[feature] Refactor lexer and parser API

[ianlewis]

I want to refactor the CustomLexer and Parser APIs so that the methods indended to be used by the lexer functions and parser functions aren't on the lexer and parser objects themselves.

Probably I'll move them to some kind of context type.

[ianlewis]

Still need to update the parser.

[ianlewis]

Need to update documentation.