Add warning capabilities to both the parser and pretty-printer

[patrickt]

Certain aspects of parsing (like parsing long int int x;) cause information to be dropped silently on the floor. This is bad. Trifecta can probably give us a nicely-formatted warning. Conversely, there are certain aspects of pretty-printing that should warn, or outright fail: we probably need to use Kmett's free-monad pretty-printer and use that in a monad transformer stack, or an extensible-effects collection.