doc/go1.2.html: first cut

Lay out the doc and write text for the minor changes.
(I left the net ones for someone who understands them better,
or for someone to describe them to me better so I can write them.)
Much still to do.

Delete go1.2.txt so there's only one thing to update.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/13597044

doc/go1.2.txt

-This file collects notes about what has changed since Go 1.1
-and should be mentioned in the Go 1.2 release notes.
-During the Go 1.2 release process it will be necessary to convert
-it to HTML, similar to go1.1.html, but for now it is a text file,
-to make the process of keeping it up-to-date more lightweight.
-
-Please keep the descriptions to a single line, starting with the
-package or cmd/xxx directory name, and ending in a CL number.
-Please keep the list sorted (as in sort.Strings of the lines).
-
-Performance:
-compress/bzip2: faster decompression by 30% (CL 9915043).
-crypto/des: 5x faster encoding/decoding (CL 11874043, 12072045).
-encoding/json: faster encoding (CL 9129044).
-net: improve windows performance by up to 30% (CL 8670044).
-net: improve performance on BSD by up to 30% (CL 8264043, 12927048, 13080043).
-
-Breaking change:
-archive/tar,archive/zip: fix os.FileInfo implementation to provide base name only (CL 13118043).
-
-cmd/5a: removed support for R9/R10 (use m/g instead) (CL 9840043).
-cmd/5l: add MOVBS, MOVHS etc for sub-word moves (CL 12682043).
-cmd/5l: support for external linking for linux/arm (CL 12871044).
-cmd/cgo, cmd/go: support including C++ code with cgo (CL 8248043).
-cmd/gc: three-index slicing to set cap as well as length (CL 10743046).
-cmd/gc: make missing package error fatal (CL 12677043).
-cmd/go: test coverage (CL 10413044).
-cmd/go: add -t flag to 'go get' to download test dependencies (CL 12566046).
-cmd/go: delete 'go doc' (CL 12974043).
-
-archive/zip: add File.DataOffset accessor (CL 12784045).
-bufio: add Reset methods to Reader and Writer (CL 12603049).
-compress/bzip2: support concatenated files (CL 12387044).
-compress/flate: add Reset method on Writer (CL 12265043).
-compress/gzip: add Reset method on Writer (CL 13435043).
-container/heap: added Fix (CL 12265043).
-container/list: added MoveBefore and MoveAfter (CL 12021044).
-crypto/cipher: AES-GCM mode (CL 12375043).
-crypto/md5: Sum function to simplify hashing (CL10624044).
-crypto/sha1: Sum function to simplify hashing (CL 10571043).
-crypto/sha256: Sum256 and Sum224 functions to simplify hashing (CL 10629043).
-crypto/sha512: Sum512 and Sum384 functions to simplify hashing (CL 10630043).
-crypto/x509: add support for reading and writing arbitrary extensions (CL 12056043).
-crypto/tls: add support for TLS 1.1, 1.2 and AES-GCM. (CL 7872043, 10762044 and 13249044).
-database/sql: add SetMaxOpenConns method on DB (CL 10726044).
-encoding: new package defining generic encoding interfaces (CL 12541051).
-encoding/csv: always allow trailing commas (CL 12294043).
-encoding/gob: support generic encoding interfaces (CL 12681044).
-encoding/json: accept but correct invalid UTF-8 in Marshal (CL 11211045).
-encoding/json: always escape ampersands (CL 12708044).
-encoding/json: support generic encoding interfaces (CL 12703043).
-encoding/xml: allow attributes stored in pointers to be marshaled (CL 8653047).
-encoding/xml: add Marshaler, MarshalerAttr interfaces (CL 12919043).
-encoding/xml: add Unmarshaler, UnmarshalerAttr interfaces (CL 12556043).
-encoding/xml: support generic encoding interfaces (CL 12751045).
-flag: add Getter interface (CL 10472043).
-flag: export commandLine (now CommandLine) (CL 12587043).
-fmt: indexed access to arguments in Printf etc. (CL 9680043).
-go/build: support including C++ code with cgo (CL 8248043).
-go/build: add Package.AllTags (CL 12703044).
-image/draw: added Drawer, FloydSteinberg and the op.Draw method (CL 10977043).
-image/draw: added Quantizer type (CL 11148043).
-image/gif: added Encode and EncodeAll (CL 10896043).
-io: Copy prioritizes WriterTo over ReaderFrom (CL 9462044).
-net: new build tag netgo for building a pure Go net package (CL 7100050).
-net/http: don't allow sending invalid cookie lines (CL 12204043).
-net/http: allow ReadResponse with nil *Request parameter (CL 9821043).
-net/http: allow responses to HEAD requests, detect type and length (CL 12583043).
-runtime: relax constraint on finalizer func in SetFinalizer (CL 12895043).
-runtime: preemption of goroutines at function entry (CL 12371043).
-sort: new Stable function provides stable sort (CL 9612044).
-strings: add IndexByte, for consistency with bytes package (CL 12214044).
-sync/atomic: add Swap functions (CL 12670045).
-syscall: implemented Sendfile for Darwin, added Syscall9 for Darwin/amd64 (CL 10980043).
-testing: AllocsPerRun is now quantized to an integer (the type is still float64) (CL 9837049).
-testing: add TB interface (intersection of T and B's methods) (CL 12962043).
-text/template: add comparison functions (CL 13091045).
-text/template: dereference pointer values when evaluating args escape funcs (CL 13257043).
-text/template: allow {{else if ... }} to simplify if chains (CL 13327043).
-time: Allow Parse and Format to handle time zone offsets with seconds (CL 8132044)
-time: patterns require non-lowercase letter to follow Mon, Jan etc (CL 12448044).
-unicode: add In, a nicer-to-use but equivalent version of IsOneOf (CL 11672044).