This finally takes the gloves off, and performs the first actual
generalization in order to implement #9586. This re-exports the
respective definitions for the 4 combinators defined in Data.Foldable.

This way, importing Data.Foldable and Control.Monad unqualified won't bring
conflicting definitions of those 4 entities into scope anymore.

This change seems to have some minor effect on rule-firing, which
causes some wibble in the test-case T4007

Reviewed By: ekmett, austin

Differential Revision: https://phabricator.haskell.org/D226