This makes sure that we catch all leftover instances of using SYB to do generic operations that we should be using Polyplate for instead. Most modules should only import Data, and possibly Typeable.
Instead of storing the Constr, which was messy, we now store a String (to allow comparison of constructor types during unification) and a function to reform the type at the end of the type checking.
As part of this patch I also had to provide a Data instance for TypeExp (to allow CompState to still be an instance of Data). Using IORefs is easier than STRef RealWorld, and puts everything in terms of IO (which is already in PassM) rather than ST (which would require more lifting).
