Parser: More comments, make lily_parser into parser_loop.
Parser: More comments, make lily_parser into parser_loop.

That function should not be called directly because the caller may not set a
raiser jump, and the caller may not have set a file. Better to let it only be
called through exported API stuff.

boardwalk/lily,crasm/lily,crasm/lily,crasm/lily,boardwalk/lily