Cache coercion kinds and roles

Because we never inspect the structure of a coercion, it is easy to cache coercions' kinds and role at the top level, using something like this:

data Coercion = MkCoercion { coercionKind :: Pair Type
                           , coercionRole :: Role
                           , coercionRep  :: CoercionRep }
data CoercionRep = -- the long, recursive datatype we have now

In the process, various role and type information currently stored in the recursive structure could be dropped.

This would make #8095 much easier.

See also #11735.