Unfortunately, this requires introducing a couple of .hs-boot files to
break up import cycles (mostly to provide class & typenames in order to
be able to write type signatures).

This does not yet re-export `(<>)` from Prelude (while the class-name
`Semigroup` is reexported); that will happen in a future commit.

Test Plan: local ./validate passed

Reviewers: ekmett, austin, bgamari, erikd, RyanGlScott

Reviewed By: ekmett, RyanGlScott

GHC Trac Issues: #14191

Differential Revision: https://phabricator.haskell.org/D3927