add support for nested gen-binds in match patterns (sort of)

needs a version of racket that supports syntax-local-match-introduce
(one that includes https://github.com/plt/racket/commit/0b045ba77b3095f53a7f4e1deb5c82b9232e17f1)
although it doesn't break the other stuff if it doesn't

Edit: Actually now for some reason it's not working anymore.