A pickle is considered as invalid if it tries to return MARK as the
result by both Python2 and Python3, e.g.:

        In [2]: pickle.loads(b"(.")
        ---------------------------------------------------------------------------
        UnpicklingError                           Traceback (most recent call last)
        <ipython-input-2-0c142c82b126> in <module>()
        ----> 1 pickle.loads(b"(.")

        UnpicklingError: unexpected MARK found

However until now, despite mark is unexported ogórek type, we were
allowing for it to be returned just ok.

The problem was caught by decode/encode roundtrip fuzz tests, e.g.

	"(Q."

panic: protocol 1: decode·encode != identity:
have: ogórek.Ref{Pid:map[interface {}]interface {}{}}
want: ogórek.Ref{Pid:ogórek.mark{}}

goroutine 1 [running]:
github.com/kisielk/og-rek.Fuzz(0x7fbe6c15f000, 0x3, 0x200000, 0x3)
        /tmp/go-fuzz-build697921479/gopath/src/github.com/kisielk/og-rek/fuzz.go:87 +0x604
go-fuzz-dep.Main(0x524d78)
        /tmp/go-fuzz-build697921479/goroot/src/go-fuzz-dep/main.go:49 +0xad
main.main()
        /tmp/go-fuzz-build697921479/gopath/src/github.com/kisielk/og-rek/go.fuzz.main/main.go:10 +0x2d
exit status 2