Refactor Lexer to avoid lex_string_factory

[jg-rp]

This PR replaces lex_string_factory with accept_string_literal and moves all state functions (lex_*) to be methods of Lexer instead of functions.

I have thought about using functools.partialmethod instead, but I'm not sure that's much better than the factory function.

The one downside to this approach is that accept_string_literal is not a true state function, but it does emit a token, which doesn't feel very neat.