Ogórek currently represents unpickled dict via map[any]any, which is
logical, but also exhibits issues because builtin Go map behaviour is
different from Python's dict behaviour. For example:

- Python's dict allows tuples to be used in keys, while Go map
  does not (https://github.com/kisielk/og-rek/issues/50),

- Python's dict allows both long and int to be used interchangeable as
  keys, while Go map does not handle *big.Int as key with the same
  semantic (https://github.com/kisielk/og-rek/issues/55)

- Python's dict allows to use numbers interchangeable in keys - all int
  and float, but on Go side int(1) and float64(1.0) are considered by
  builtin map as different keys.

- In Python world bytestring (str from py2) is considered to be related
  to both unicode (str on py3) and bytes, but builtin map considers all
  string, Bytes and ByteString as different keys.

- etc...

All in all there are many differences in behaviour in builtin Python
dict and Go map that result in generally different semantics when
decoding pickled data. Those differences can be fixed only if we add
custom dict implementation that mirrors what Python does.

-> Do that: add custom Dict that implements key -> value mapping with
   mirroring Python behaviour.

For now we are only adding the Dict class itself and its tests.
Later we will use this new Dict to handle decoding dictionaries from the pickles.

For the implementation we use github.com/aristanetworks/gomap which
provides extraction of builtin go map code wrapped into generic type
Map[Key,Value] that accepts custom equal and hash functions. And it is
those equal and hash functions via which we define equality behaviour to
be the same as on Python side.

Please see documentation in the added code and tests for details.

In the next patch we will need to use generics, which are only available
starting from Go 1.18 . That Go 1.18 was released in spring of 2022 -
almost 2.5 years ago - and is already EOL now. The older releases became
EOL even earlier compared to Go 1.18 and are generally no longer
supported by Go upstream. Dropping support for Go < 1.18 remains support
for Go 1.18 - 1.23, i.e. 6 releases 3 of which are already in end of
life state. So dropping support for Go < 1.18 should be, hopefully, ok.

As recent fixes show it is easy to break benchmarks while adjusting
tests without noticing. Prevent that to happen by running all benchmarks
just once during each CI run.

Under those go versions `go test` does not support `-benchtime Nx`
syntax, that we are going to use in the next patch, and fails with

    invalid value "1x" for flag -test.benchtime: time: unknown unit x in duration 1x

Maybe it is not a big deal and if it were just `-benchtime Nx` we could
try to invent and do something to try to support those old Go versions.
However I have upcoming patches that will require Go 1.18 for generics,
and from that point of view trying to keep support for Go version older
than Go 1.18 would be not practical.

After 2c359fc1 (decoder: Remember protocol version as last seen in a
PROTO opcode) BenchmarkDecode started to fail with

    --- FAIL: BenchmarkDecode
        ogorek_test.go:977: invalid pickle version

That happens because some input pickles now come with `PROTO 0xff`
prefix and version 0xff is indeed invalid.

-> Fix it by adjusting that prefix to be `PROTO 3` during benchmark as 3
is the most-widely used protocol version used by both py3 and py2 (via
zodbpickle).

After 6e5e403e (encoder: Fix GLOBAL emission wrt module/name with \n)
BenchmarkEncode started to fail with

    --- FAIL: BenchmarkEncode
        ogorek_test.go:1003: protocol 0-3: global: module & name must be string without \n

-> Fix it by taking as input only those testdata objects that are marked
to encode without errors.

BININT1 emits unsigned integer that fits into 1 byte.
BININT2 emits unsigned integer that fits into 2 bytes.

So they cover [0, 0xff] and [0, 0xffff] ranges including edges. However
we were emitting BININTX opcodes only for numbers being strictly inside
those ranges - i.e. (0, 0xff) and (0, 0xffff) - without edges. As the
result what could be emitted as "K\x00." was emitted as
"J\x00\x00\x00\x00." and so on.

Fix it by emitting BININT1 for [0, 0xff] inclusive and BININT for [0,
0xffff] also inclusive.

BINUNICODE opcode comes with 4-byte little-endian unsigned int length after it.
However we were decoding it as signed int32 instead of unsigned uint32.

As the result if last byte of this length is >= 0x80 the length
was decoded as negative:

        y := int32(0x81)
        println(y << (3*8))

	Output: -2130706432

which was leading to empty unicode string returned instead of an error.

-> Fix it by decoding BINUNICODE lenght as uint32.

Without the fix added test fails as

	--- FAIL: TestDecodeError (0.00s)
	    ogorek_test.go:812: "X\xff\xff\xff\xff.": no decode error  ; got "", <nil>

And for that "X\xff\xff\xff\xff." Python also rejects to load this
pickle:

	In [1]: p = b"X\xff\xff\xff\xff."

	In [2]: pickle.loads(p)
	---------------------------------------------------------------------------
	UnpicklingError                           Traceback (most recent call last)
	Cell In [2], line 1
	----> 1 pickle.loads(p)

	UnpicklingError: pickle data was truncated

	In [3]: dis(p)
	---------------------------------------------------------------------------
	ValueError                                Traceback (most recent call last)
	Cell In [3], line 1
	----> 1 dis(p)

	File /usr/lib/python3.11/pickletools.py:2448, in dis(pickle, out, memo, indentlevel, annotate)
	   2446 errormsg = None
	   2447 annocol = annotate  # column hint for annotations
	-> 2448 for opcode, arg, pos in genops(pickle):
	   2449     if pos is not None:
	   2450         print("%5d:" % pos, end=' ', file=out)

	File /usr/lib/python3.11/pickletools.py:2291, in _genops(data, yield_end_pos)
	   2289     arg = None
	   2290 else:
	-> 2291     arg = opcode.arg.reader(data)
	   2292 if yield_end_pos:
	   2293     yield opcode, arg, pos, getpos()

	File /usr/lib/python3.11/pickletools.py:693, in read_unicodestring4(f)
	    691 if len(data) == n:
	    692     return str(data, 'utf-8', 'surrogatepass')
	--> 693 raise ValueError("expected %d bytes in a unicodestring4, but only %d "
	    694                  "remain" % (n, len(data)))

	ValueError: expected 4294967295 bytes in a unicodestring4, but only 1 remain

Fix noticed typos, clarify things in a couple of places. Only minor
and documentation changes.

/helped-by @kisielk

Rerunning go-fuzz after recent patches resulted in some new fuzz test vectors.

Up till now ogórek works relatively well for me but experience gained
with ZODB/go and Wendelin.core highlighted two problems with strings:

1) loading, then re-saving string data is not generally identity, and
2) there is no way to distinguish binary data saved via py2 str from
   unicode strings saved via either py2 unicode or py3 str.

Let me explain those problems:

1) Loading, then re-saving string data is not generally identity
----------------------------------------------------------------

On decoding ogórek currently loads both byte-string (*STRING opcodes)
and unicode string (*UNICODE opcodes) into the same Go type string. And
on encoding that string type is encoded as *STRING for protocol <= 2 and
as *UNICODE for protocol >= 3. It was me to implement that encoding
logic in 2018 in e7d96969 (encoder: Fix string wrt protocol version)
where in particular I did

    - if protocol >= 3 we have to emit the string as unicode pickle object
      the same way as Python3 does. If we don't do - Python3 won't be
      generally able to load our pickle ...

with the idea that protocol=3 always means that the pickle is intended
to be for Python3.

But there I missed that zodbpickle can use and generate pickles with
protocol=3 even on py2 and that ZODB/py2 actually uses this protocol=3
mode since https://github.com/zopefoundation/ZODB/commit/12ee41c4
authored in the same 2018.

So, there can be pickles saved under protocol=3 that do contain both
*STRING and *UNICODE opcodes, and when ogórek sees those it loads that
*STRING and *UNICODE data as just string loosing the information about
type of particular variant. And then on encoding the data is saved as
all UNICODE, or all STRING, thus breaking decode/encode=identity
property.

This breakage is there even with plain pickle and protocol=2 - when both
*STRING and *UNICODE are in the database, ogórek loads them both as the
same Go type string, and then saving back under the same protocol=2 goes
as all STRING resulting in unicode objects becoming str on resave
without any intended change.

2) there is no way to distinguish binary data saved via py2 str from unicode strings saved via either py2 unicode or py3 str
----------------------------------------------------------------------------------------------------------------------------

Continuing above example of py2 database with both *STRING and *UNICODE
opcodes present there is currently no way for application to distinguish
those from each other. In other words there is currently no way for the
application to distinguish whether it is binary data coming from py2
protocol <= 2 era, from unicode text.

The latter problem I hit for real: with Wendelin.core we have lots of
data saved from under Python2 as just str. And the Go part of
Wendlin.core, upon loading block of data, wants to accept only binary -
either bytes from py3 or bytestring from py2, but not unicode, because
it indicates a mistake if e.g. a ZBlk object would come with unicode data

https://lab.nexedi.com/nexedi/wendelin.core/-/blob/07087ec8/bigfile/file_zodb.py#L267-300
https://lab.nexedi.com/nexedi/wendelin.core/-/blob/07087ec8/wcfs/internal/zdata/zblk.go#L31-32
https://lab.nexedi.com/nexedi/wendelin.core/-/blob/07087ec8/wcfs/internal/zdata/zblk.go#L98-107

but there is currently no way to distinguish whether it was unicode or
bytestring saved into the database becase they both are represented as
the same Go string type after decoding.

----------------------------------------

So to solve those problems I thought it over and understood that the
issues start to appear becase we let *STRING and *UNICODE to become
mixed into the same entity on loading. This behaviour is there from
ogórek beginning and the intention, it seems, was to get the data from
the pickle stream in an easily-accepted form on the Go side. However the
convenience turned out to come with cost of loosing some correctness in
the general case as explained above.

So if we are to fix the correctness we need to change that behaviour and
load *STRING and *UNICODE opcodes into distinct types, so that the
information about what was what is preserved and it becomes possible to
distinguish bytestring from unicode strings and resave the data in
exactly the same form as loaded. Though we can do this only under opt-in
option with default behaviour staying as it was before to preserve
backward compatibility.

-> Do it.

Below is excerpt from doc.go and DecoderConfig and EncoderConfig changes that
describe the new system:

    For strings there are two modes. In the first, default, mode both py2/py3
    str and py2 unicode are decoded into string with py2 str being considered
    as UTF-8 encoded. Correspondingly for protocol ≤ 2 Go string is encoded as
    UTF-8 encoded py2 str, and for protocol ≥ 3 as py3 str / py2 unicode.
    ogórek.ByteString can be used to produce bytestring objects after encoding
    even for protocol ≥ 3. This mode tries to match Go string with str type of
    target Python depending on protocol version, but looses information after
    decoding/encoding cycle:

        py2/py3 str  ↔  string                       StrictUnicode=n mode, default
        py2 unicode  →  string
        py2 str      ←  ogórek.ByteString

    However with StrictUnicode=y mode there is 1-1 mapping in between py2
    unicode / py3 str vs Go string, and between py2 str vs ogórek.ByteString.
    In this mode decoding/encoding and encoding/decoding operations are always
    identity with respect to strings:

        py2 unicode / py3 str  ↔  string             StrictUnicode=y mode
        py2 str                ↔  ogórek.ByteString

    For bytes, unconditionally to string mode, there is direct 1-1 mapping in
    between Python and Go types:

        bytes        ↔  ogórek.Bytes   (~)
        bytearray    ↔  []byte

    --------

    type DecoderConfig struct {
        // StrictUnicode, when true, requests to decode to Go string only
        // Python unicode objects. Python2 bytestrings (py2 str type) are
        // decoded into ByteString in this mode...
        StrictUnicode bool
    }

    type EncoderConfig struct {
        // StrictUnicode, when true, requests to always encode Go string
        // objects as Python unicode independently of used pickle protocol...
        StrictUnicode bool
    }

Since strings are now split into two types, string and ByteString, and
ByteString can either mean text or binary data, new AsString and AsBytes
helpers are also added to handle string and binary data in uniform way
supporting both py2 and py3 databases. Corresponding excerpts from
doc.go and typeconv.go changes with the description of those helpers
come below:

    On Python3 strings are unicode strings and binary data is represented by
    bytes type. However on Python2 strings are bytestrings and could contain
    both text and binary data. In the default mode py2 strings, the same way as
    py2 unicode, are decoded into Go strings. However in StrictUnicode mode py2
    strings are decoded into ByteString - the type specially dedicated to
    represent them on Go side. There are two utilities to help programs handle
    all those bytes/string data in the pickle stream in uniform way:

        - the program should use AsString if it expects text   data -
          either unicode string, or byte string.
        - the program should use AsBytes  if it expects binary data -
          either bytes, or byte string.

    Using the helpers fits into Python3 strings/bytes model but also allows to
    handle the data generated from under Python2.

    --------

    // AsString tries to represent unpickled value as string.
    //
    // It succeeds only if the value is either string, or ByteString.
    // It does not succeed if the value is Bytes or any other type.
    //
    // ByteString is treated related to string because ByteString represents str
    // type from py2 which can contain both string and binary data.
    func AsString(x interface{}) (string, error) {

    // AsBytes tries to represent unpickled value as Bytes.
    //
    // It succeeds only if the value is either Bytes, or ByteString.
    // It does not succeed if the value is string or any other type.
    //
    // ByteString is treated related to Bytes because ByteString represents str
    // type from py2 which can contain both string and binary data.
    func AsBytes(x interface{}) (Bytes, error) {

ZODB/go and Wendelin.core intend to switch to using StrictUnicode mode
while leaving ogórek to remain 100% backward-compatible in its default
mode for other users.

On Python two different objects with different types can represent
essentially the same entity. For example 1 (int) and 1L (long) represent
integer number one via two different types and are decoded by ogórek into Go
types int64 and big.Int correspondingly. However on the Python side those
two representations are often used interchangeably and programs are usually
expected to handle both with the same effect.

For example I hit this situation for real with FileStorage index, ZEO
protocol, BTrees and Wendelin.core where data with integer fields, for
the same field, are sometimes represented as int and sometimes as long.
As the result ZODB/go and Wendelin.core had private XInt64 helper used
as follows:

https://lab.nexedi.com/nexedi/wendelin.core/-/blob/07087ec8/wcfs/internal/pycompat/pycompat.go
https://lab.nexedi.com/nexedi/wendelin.core/-/blob/07087ec8/wcfs/internal/zdata/zblk.go#L385
https://lab.nexedi.com/search?utf8=%E2%9C%93&search=XInt64&group_id=&project_id=73&search_code=true&repository_ref=f7776fc1&nav_source=navbar

Besides int/long we are also hitting similar situation with
bytes / str(py2) / str(py3) which will also need to use corresponding
helpers to be able to load pickled data from existing databases.

In the next patch we will add StrictUnicode mode to handle that strings,
and it will naturally come with AsBytes and AsString helpers. Thus it
also feels appropriate to keep the other useful helpers to handle
unpickled data inside ogórek itself.

This patch adds AsInt64 helper with top-level description and tests.

In the next patch I will need to use big.Int.IsInt64, which was added in
Go 1.9 in 2017. Go 1.7 and Go 1.8 are very old nowdays - their last
point releases were done in 2017 (go1.7.6) and 2018 (go1.8.7) - 7 and 6
years ago correspondingly. Since then those releases are EOL with many
further Go releases done after them. We even specify go1.12 as the
requirement in our go.mod, but that is probably because go1.12 is the
first version to support modules. Anyway dropping support for those very
old Go1.7 and Go1.8 should be almost certainly ok as I doubt if there
are current ogórek users still stuck at those old Go versions.

In 9daf6a2a (Fix UNICODE decoding) I fixed protocol 0 UNICODE decoding
by implementing "raw-unicode-escape" decoder and switching unpickler to
use that to match 1-to-1 what Python unpickler does.

In 57f875fd (encoder: Fix protocol 0 UNICODE emission) I further fixed
protocol 0 UNICODE encoding by implementing "raw-unicode-escape" encoder
and switching pickler to use that, trying to match 1-to-1 what Python
pickler does.

However there is a difference in between Python unicode and unicode on
Go side: in Python unicode is immutable array of UCS code points. On Go
side, unicode is immutable array of bytes, that, similarly to Go string
are treated as being mostly UTF-8. We did not pick e.g. []rune to
represent unicode on Go side because []rune is not immutable and so
cannot be used as map keys.

So Go unicode can be either valid UTF-8 or invalid UTF-8. For valid
UTF-8 case everything is working as intended: our raw-unicode-escape
decoder, because it works in terms of UCS, can produce only valid UTF-8,
while our raw-unicode-escape encoder also matches 1-to-1 what python
does because for valid UTF-8 utf8.DecodeRuneInString gives good rune and
the encoder further works in terms of UCS.

However for the case of invalid UTF-8, there is a difference in between
what Python and Go raw-unicode-escape encoders do: for Python this case
is simply impossible because there input is []UCS. For the Go case, in
57f875fd I extended the encoder to do:

	// invalid UTF-8 -> emit byte as is
	case r == utf8.RuneError:
		out = append(out, s[0])

which turned out to be not very thoughtful and wrong because
original raw-unicode-escape also emits UCS < 0x100 as those plain bytes
and so if we also emit invalid UTF-8 as is then the following two inputs
would be encoded into the same representation "\x93":

	unicode("\x93")     // invalid UTF-8
	unicode("\xc2\x93)  // UTF-8 of \u93

which would break `decode/encode = identity` invariant and corrupt the
data because when loaded back it will be "\xc2\x93" instead of original "\x93".

-> Fix it by rejecting to encode such invalid UTF-8 via protocol 0 UNICODE.

Unfortunately rejection is the only reasonable choice because
raw-unicode-escape codec does not allow \xAA to be present in the output
stream and so there is simply no way to represent arbitrary bytes there.

Better to give explicit error instead of corrupting the data.

For protocols ≥ 1 arbitrary unicode - both valid and invalid UTF-8 - can
still be loaded and saved because *BINUNICODE opcodes come with
bytestring argument and there is no need to decode/encode those
bytestrings.

/reviewed-by @kisielk
/reviewed-on https://github.com/kisielk/og-rek/pull/67

For example currently if result of decoding is expected to be unicode,
but is actually string the following confusing error is generated with
exactly the same have and want:

    --- FAIL: TestDecode/unicode(non-utf8)/"X\x01\x00\x00\x00\x93." (0.00s)
        ogorek_test.go:504: decode:
            have: "\x93"
            want: "\x93"

This happens because by default Printf("%#v") does not emit object type
even if it was derived type from builtin string - either our Bytes or
unicode, and emits the same string representation irregardless of which
type it is.

I think out-of-the box behaviour of %#v is unreasonable, but it is there
for a long time and so instead of trying to argue with Go upstream,
which after many years of arguing I think is not practical, teach our
Bytes and unicode types how to represent themselves under %#v with
indicating their type in the output.

After the fix the error from above becomes

    --- FAIL: TestDecode/unicode(non-utf8)/"X\x01\x00\x00\x00\x93." (0.00s)
        ogorek_test.go:504: decode:
            have: "\x93"
            want: ogórek.unicode("\x93")

which is more clear because want is different from have.

Adjusted behaviour is not test-only, because user applications might
want to do the same %#v at application level and there it would be also
confusing not to see a type.

/reviewed-by @kisielk
/reviewed-on https://github.com/kisielk/og-rek/pull/67

fuzz: Export tests pickles to fuzz/corpus with stable filenames

In 9d1344ba (fuzz: Automatically export all tests pickles into
fuzz/corpus) I added code that automatically exports all test pickles
to fuzz corpus, so that fuzzing starts from all the tricky cases we
already put into tests. However exported files were named as
test-i-j.pickle, where i is sequential number of test entry in global
test vector. This way if we insert some new entry in the middle, or
change entries order it causes lots of renaming inside fuzz/corpus/ on
next `go generate`.

-> Fix that by emitting vectors as test-<sha1-of-pickle>.pickle so that
file name depends only on pickle content are remains stable
independently of entry position in the list of tests.

Besides changes to fuzz_test.go this patch brings:

1) rm fuzz/corpus/test-*.pickle
2) `go generate`

The result inside fuzz/corpus/ is pure renaming + removal of duplicates.
For example test-15-4.pickle is removed because the same data was in test-14-4.pickle
and now resides in test-ee4459c0c165c4cbc3672ee5ef5438e00121b229.pickle.

decoder: Fix integer overflow in BYTEARRAY8 handling

Rerunning recent go-fuze resulted in adding some new test vectors.

Rerunning fuzzing afresh found the following crash because uint64 len > max(int64)
was cast to int resulting in negative number:

    "\x960000000\xbd"

    panic: bytes.Buffer.Grow: negative count

    goroutine 1 [running]:
    bytes.(*Buffer).Grow(0xc000012140?, 0xc00010abf8?)
            /home/kirr/src/tools/go/go1.22/src/bytes/buffer.go:166 +0xb4
    github.com/kisielk/og-rek.(*Decoder).bufLoadBytesData(0xc000072000, 0xbd30303030303030)
            /home/kirr/src/neo/src/github.com/kisielk/og-rek/ogorek.go:788 +0xd5
    github.com/kisielk/og-rek.(*Decoder).bufLoadBinData8(0xc000072000)
            /home/kirr/src/neo/src/github.com/kisielk/og-rek/ogorek.go:776 +0x105
    github.com/kisielk/og-rek.(*Decoder).loadBytearray8(0xc000072000)
            /home/kirr/src/neo/src/github.com/kisielk/og-rek/ogorek.go:1270 +0x34
    github.com/kisielk/og-rek.(*Decoder).Decode(0xc000072000)
            /home/kirr/src/neo/src/github.com/kisielk/og-rek/ogorek.go:311 +0x1a2c
    github.com/kisielk/og-rek.Fuzz({0x7fd052187000, 0x9, 0x9})
            /home/kirr/src/neo/src/github.com/kisielk/og-rek/fuzz.go:15 +0xf6
    go-fuzz-dep.Main({0xc00010af38, 0x1, 0x5bc338?})
            go-fuzz-dep/main.go:36 +0x14c
    main.main()
            github.com/kisielk/og-rek/go.fuzz.main/main.go:15 +0x35
    exit status 2

-> Fix it by first comparing uint64 len without casting and further casting to
int only after we cap the len to be in safe range.

* Switch from Travis CI to GitHub Actions

Travis CI no longer works for ogórek and https://travis-ci.org/kisielk/og-rek
returns "not found".

-> Switch to GitHub Actions to fix CI.

We loose CI coverage for "go tip" as specifying "tip" Go version in
actions/setup-go is not supported out of the box:

https://github.com/actions/setup-go/issues/21

Maybe in 201x it was relevant to test wrt go tip, but nowdays I think
this is no longer much needed.

* CI += go1.16 - go1.22

Add CI coverage for all releases past go1.15 that we had before.

Pickle protocol 5 adds out-of-band data and BYTEARRAY8 opcode (see [1]
and [2]). We add support for BYTEARRAY8 here. Handling out-of-band data
would require more setup from users - on both decoding and encoding
side, and it is likely that currently no practical use-case exists to
work with pickles with out-of-band data on Go side. This way we
behave as if no out-of-band data was provided when seeing all protocol 5
opcodes besides BYTEARRAY8.

Hopefully fixes: https://github.com/kisielk/og-rek/issues/62

[1] https://www.python.org/dev/peps/pep-0574
[2] https://github.com/python/cpython/commit/91f4380cedba

/reviewed-by @kisielk
/reviewed-on https://github.com/kisielk/og-rek/pull/63

Ogórek can be already used from another module (but at outdated
version), but let's be explicit about that. Go.mod was created by `go
mod init github.com/kisielk/og-rek`. There is no external dependencies.

Go1.14 and Go1.15 are the current stable Go releases.
Go1.12 and Go1.14 fill in the gap in testing coverage in between -go1.11
and current go releases.

Maybe we can eventually drop older go release from travis coverage.

Add brief overview on to how to use ogórek, and how Python and Go types
are mapped with each other. For completeness give overview of pickle
protocols and overview of persistent references.

(Documentation put into separate doc.go as suggested by Kamil)

Rerun `go generate` since new tests vectors have been added in recent
bytes/bytearray patches.

Starting from 2013 (from 555efd8f "first draft of dumb pickle encoder")
wrt []byte ogórek state was:

1. []byte was encoded as string
2. there was no way to decode a pickle object into []byte

then, as

- []byte encoding was never explicitly tested,
- nor I could find any usage of such encodings via searching through all Free /
  Open-Source software ogórek uses - I searched via "Uses" of NewEncoder on godoc:

  https://sourcegraph.com/github.com/kisielk/og-rek/-/blob/encode.go#L48:6=&tab=references:external

it is likely that []byte encoding support was added just for the sake of
it and convenience and then never used. It is also likely that the
original author does not use ogórek encoder anymore:

	https://github.com/kisielk/og-rek/pull/52#issuecomment-423639026

For those reasons I tend to think that it should be relatively safe to
change how []byte is handled:

- the need to change []byte handling is that currently []byte is a kind of
  exception: we can only encode it and not decode something into it.
  Currently encode/decode roundtrip for []byte gives string, which breaks
  the property of encode/decode being identity for all other basic types.

- on the similar topic, on encoding strings are assumed UTF-8 and are
  encoded as UNICODE opcodes for protocol >= 3. Passing arbitrary bytes
  there seems to be not good.

- on to how change []byte - sadly it cannot be used as Python's bytes
  counterpart. In fact in the previous patch we actually just added
  ogórek.Bytes as a counterpart for Python bytes. We did not used []byte
  for that because - contrary to Python bytes - []byte cannot be used as a
  dict key.

- the most natural counterpart for Go's []byte is thus Python's
  bytearray:

	https://docs.python.org/3/library/stdtypes.html#bytearray-objects

  which is "a mutable counterpart to bytes objects"

So add Python's bytearray decoding into []byte, and correspondingly
change []byte encoding to be encoded as bytearray.

P.S.

This changes encoder semantic wrt []byte. If some ogórek use breaks
somewhere because of it, we could add an encoder option to restore
backward compatible behaviour. However since I suspect noone was
actually encoding []byte into pickles, I prefer we wait for someone to
speak-up first instead of loading EncoderConfig with confusion options
that nobody will ever use.

In the next patch decoding a pickle will depend on whether current
protocol is <= 2 or >= 3. For this let's teach PROTO opcode handler to
recall last seen protocol version.

For testing - prepare tests infrastructure for cases where protocol
version affects decoding semantic: if a test pickle in tests table comes
with \x80\xff prefix, on decode tests this prefix will be changed to
concrete

	\x80 ver

with all versions protperly iterated as specified in TestPickle.protov.

We will also need this functionality in the next patch.

In Python bytes is immutable and read-only array of bytes. It is
also hashable and so is different from go []byte in that it can be
used as a dict key. Thus the closes approximation for Python bytes in Go
is some type derived from Go's string - it will be different from string
and at the same time will inherit from string it immutability property
and being able to be used as map key. So

- add ogórek.Bytes type to represent Python bytes
- add support to decode BINBYTES* pickle opcodes (these are protocol 3 opcodes)
- add support to encode ogórek.Bytes via those BINBYTES* opcodes
- for protocols <= 2, where there is no opcodes to directly represent
  bytes, adopt the same approach as Python - by pickling bytes as

	_codecs.encode(byt.decode('latin1'), 'latin1')

  this way unpickling it on Python3 will give bytes, while unpickling it
  on Python2 will give str:

	In [1]: sys.version
	Out[1]: '3.6.6 (default, Jun 27 2018, 14:44:17) \n[GCC 8.1.0]'

	In [2]: byt = b'\x01\x02\x03'

	In [3]: _codecs.encode(byt.decode('latin1'), 'latin1')
	Out[3]: b'\x01\x02\x03'

  ---

	In [1]: sys.version
	Out[1]: '2.7.15+ (default, Aug 31 2018, 11:56:52) \n[GCC 8.2.0]'

	In [2]: byt = b'\x01\x02\x03'

	In [3]: _codecs.encode(byt.decode('latin1'), 'latin1')
	Out[3]: '\x01\x02\x03'

- correspondingly teach decoder to recognize particular calls to
  _codecs.encode as being representation for bytes and decode it
  appropriately.

- since we now have to emit byt.decode('latin1') as UNICODE - add, so
  far internal, `type unicode(string)` that instructs ogórek encoder to
  always emit the string with UNICODE opcodes (regular string is encoded
  to unicode pickle object only for protocol >= 3).

- For []byte encoding preserve the current status - even though
  dispatching in Encoder.encode changes, the end result is the same -
  []byte was and stays currently encoded as just regular string.

  This was added in 555efd8f "first draft of dumb pickle encoder", and
  even though that might be not a good choice, changing it is a topic for
  another patch.

Previously, we were quoting UNICODE opcode argument with strconv.QuoteToASCII().
However that function, in addition to \u and \U escapes, can produce
e.g. \n, \r, \xAA etc escapes. And all of the latter variants are not
treated as special escapes of a unicode literal by Python, thus leading
to data being wrongly received.

Fix it by doing exactly the same that Python pickle encoder does - the
UNICODE argument comes are "raw-unicode-escape" encoded.

This patch contains only codec tests - not end-to-end pickle tests,
because currently Encoder.encodeUnicode() is called only from under
Encoder.encodeString(), and there only from under

	if e.config.Protocol >= 3

We will indirectly add tests for encodeUnicode @ protocol=0 in the next
patches, while adding support for Python bytes.

I initially added pyquote only as debugging tool (in b429839d "tests:
Show pickles in a way that can be copy-pasted into Python"), and later
it started to be used in the Encoder (see 18004fbd "Move pyquote into
main codebase"). However there is no explicit tests for pyquote.

Add some pyquote tests.

Both object and objectDecodedBack are interace{}. So it could turn out,
if e.g. they are both of the same non-comparable type (e.g. []byte),
comparing them directly will panic.

I was (re-)running fuzz tests again, and in addition to what was already
known (e.g. self-referencing structures) found two new issues:

- long as dict keys (https://github.com/kisielk/og-rek/issues/55), and
- encoding global with '\n' in module name (fixed in previous patch).

Update the corpus with points go-fuzz found, so that next runs could
restart with having all the previous interesting input vectors available.

Caught via fuzzing:

	"\x8c\x030\n02\x93."

        0: \x8c SHORT_BINUNICODE '0\n0'
        5: 2    DUP
        6: \x93 STACK_GLOBAL
        7: .    STOP

	panic: protocol 0: decode back error: err
	pickle: "c0\n0\n0\n0\n."

	goroutine 1 [running]:
	github.com/kisielk/og-rek.Fuzz(0x7f2f1009a000, 0x8, 0x200000, 0x3)
	        /tmp/go-fuzz-build645492341/gopath/src/github.com/kisielk/og-rek/fuzz.go:47 +0x8b8
	go-fuzz-dep.Main(0x525e10)
	        /tmp/go-fuzz-build645492341/goroot/src/go-fuzz-dep/main.go:49 +0xad
	main.main()
	        /tmp/go-fuzz-build645492341/gopath/src/github.com/kisielk/og-rek/go.fuzz.main/main.go:10 +0x2d
	exit status 2

i.e. '0\n0' module name was emitted as-is as part ot text-based GLOBAL which
completely broke pickle stream.

For the reference Python decodes such globals with \n in name just ok:

	In [10]: s = b"S'decimal\\nq'\nS'Decimal'\n\x93."

	In [11]: pickle.loads(s)
	---------------------------------------------------------------------------
	ModuleNotFoundError                       Traceback (most recent call last)
	<ipython-input-11-764e4625bc41> in <module>()
	----> 1 pickle.loads(s)

	ModuleNotFoundError: No module named 'decimal\nq'

	In [12]: import sys

	In [15]: d = sys.modules['decimal']

	In [16]: sys.modules['decimal\nq'] = d

	In [17]: pickle.loads(s)
	Out[17]: decimal.Decimal

I put "err" into format string instead of argument. Apologize for that...

While fixing https://github.com/kisielk/og-rek/issues/48 there were
corresponding test-vector additions. Regenerate the fuzz test corpus.

UNICODE is text-based opcode, which is used at protocol 0 and follows
'raw-unicode-escape' encoded argument till EOL.

- for decoding we must explicitly implement Python's
  'raw-unicode-escape' codec decoding, which is used by Python's pickle
  for UNICODE argument.

Updates and hopefully fixes: https://github.com/kisielk/og-rek/issues/48

STRING is text-based opcode which is used at protocol 0 and follows
\-escaped argument till EOL.

- for encoding we must not use Go's %q, since that will use \u and \U
  when seeing corresponding bytes, and since Python does not interpret
  \u or \U in string literals, the data received at Python side will be
  different.

- for decoding we must explicitly implement Python's 'string-escape'
  codec decoding which is used by Python's pickle for STRING opcode
  argument.

Updates: https://github.com/kisielk/og-rek/issues/48

We added pyquote in b429839d (tests: Show pickles in a way that can be
copy-pasted into Python). However in the next patch this functionality
will be needed for the encoder to fix encoding of strings at protocol 0.
Thus we need this function to be not test-only.

Plain code movement here, no semantic change.