Massive refactor.
Moves all parse functions which require a lexer into `context_parser`.
Adds dependency on a `context` to `AST` for AST building calls.
Was going to add error handler passing to AST node resolvers; in fact, that was the point of this refactor. It's going to have to be a "next commit" thing. This one's already pretty bogged down.
Total error count remains at 650.