Investigate ConstraintKinds and type families to simplify the parser tower's type signatures

[patrickt]

I managed to add a little simplicity in d8a7e67, at the cost of turning on UndecidableInstances. It would be cool if we could avoid undecidable instances.

Another idea I had:

class (TypeParsing m) => VariableParsing m where
    type VariableSig :: Constraint
    parseVariable :: (VariableSig f) => m (Term f)