Add aliases :gb2312 :gb18030 to :gbk encoding

[eugeneia]

GB 2312, GB 18030 and GBK 1.0 are compatible encodings, see
https://en.wikipedia.org/wiki/GB_18030

[luismbo]

Does compatible imply that GB 18030 is a superset of those character sets? UTF-8 and the various ISO-8859-* encodings are also compatible with ASCII but ASCII is not (and should not be) an alias for any of those.

[eugeneia]

Difficult to tell. I found some useful information here: https://en.wikipedia.org/wiki/GB_2312#Two_implementations_of_GB2312

W3C's technical recommendation specifies a GBK encoding to be inferred for streams labelled gb2312, which in turn uses a GB18030 decoder.

As far as I understand the formats should be able to be implemented by a single encoding (the implementation of :gbk?) that maps to Unicode code points. The differences between GB* seem to be about what they “map to”, and not what they “map from”. Apparently, versions of GB* mapped to Unicode private use codepoints before the appropriate Unicode codepoints were defined by the consortion.

Since :gbk is broken as of this time (see #31), I don’t think there is a correct solution here at the moment.