Fixed derps in `grammar.h` - `lexer`, made it compile. I have to
write a converter for `integer_literal`, though. Next target:
`instruction` and `line` parser, `effective_address`, `xxx_override`
and `cpu_register` constructors. And before those constructors,
sanely typed helper parsers for use with `effective_addres`.
Probably also some kind of polymorphism-aware parser? Dunno if
it is a good idea to extend the parser this close to assembler's
deadline, but we'll see.