Define typeRep in terms of new Proxy#

Data.Typeable.Internal.Typeable now contains typeRep# - defined over
Proxy# - instead of typeRep, the latter now being a wrapper of the
former.

Authored-by: Edward Kmett <ekmett@gmail.com>
Authored-by: Austin Seipp <austin@well-typed.com>
Signed-off-by: Austin Seipp <austin@well-typed.com>