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.
classify some traits
Erase linear overhead in kleisliU.
Merge pull request #523 from xuwei-k/classify

classify some traits
==>> universal Traverse, semigroups, & Bifoldable. (#519)
MapTest imports.