Summary:
`TyCon` and `TypeRep` are supposed to be abstract, by providing these
additional few public operations the need to import
`Data.Typeable.Internal` is reduced, and future changes to the internal
structure of `TypeRep`/`TyCon` shouldn't require changes in packages such as
`deepseq` or `hashable` anymore (hopefully).

Reviewers: ekmett, simonpj, tibbe, austin

Reviewed By: ekmett, simonpj, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D699