This refactors `(>>)`/`(*>)`/`return`/`pure` methods into normal form.

The redundant explicit `return` method definitions are dropped
altogether.

The explicit `(>>) = (*>)` definitions can't be removed yet, as
the default implementation of `(>>)` is still in terms of `(*>)`
(even though that should have been changed according to the AMP but
wasn't -- see note in GHC.Base for details why this had to be postponed)

A nofib comparision shows this refactoring to result in minor runtime
improvements (unless those are within normal measurement fluctuations):

        Program           Size    Allocs   Runtime   Elapsed  TotalMem
  -------------------------------------------------------------------------
            Min          -0.0%     -0.0%     -1.6%     -3.9%     -1.1%
            Max          -0.0%     +0.0%     +0.5%     +0.5%      0.0%
  Geometric Mean         -0.0%     -0.0%     -0.4%     -0.5%     -0.0%

Full `nofib` report at https://phabricator.haskell.org/P68

Reviewers: quchen, alanz, austin, #core_libraries_committee, bgamari

Reviewed By: bgamari

Differential Revision: https://phabricator.haskell.org/D1316