Text.Gigaparsec.Token support

[j-mie6]

This one is a big one: we want to support the whole parsley.token package, including the description ADTs and the generated parsers. This is quite an undertaking, and much inspiration can be drawn from parsley, as the bare implementations are preserved in the test suites at least. I suspect there will be challenges here:

• parsley uses a global register to track the current space implementation for context-dependent whitespace, we don't have such a mechanism, so we'll need to actually support the embedding of IORefs into Reg with all the unsafe functions that come with it, sad but true. In this sense, the Lexer record will store an IORef inside.