Clean up ==>> Functor stack; ==>> is Bifoldable too.

* Add an explicit left-fold for Foldable ==>>.

* Merge all Functorish into mapTraversable, renamed mapCovariant.

* Traverse[[S]S ==>> A] doesn't need Order[S].

* Specialized fold/map will be used even if Order[S] is available.
Remove redundant ==>> tests.
Intersection semigroup & union monoid for ==>>.
Optimize #equal for ==>> Order.
Rewrite mapShow with contramap.
Merge mainline for consistency fixes since divergence.
Fix implicit ambiguity.
2.9.3 picked the wrong foldMap result type.
==>> universal Traverse, semigroups, & Bifoldable. (#519)
MapTest imports.