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