Bump mtl and transformers upper bound

I'm not sure if this breaks the world. The doctests still pass, and this gets me out of cabal hell for one of my projects and into cabal purgatory until https://github.com/ekmett/parsers/commit/a6392f3547daa3fced8268e786d506aa639419a5 makes it to hackage, but I am not sure if it has any negative consequences.