This adds `Data.Bifoldable` and `Data.Bitraversable` from the
`bifunctors` package to `base`, completing the migration started in
D336.  This is fairly straightforward, although there were a suprising
amount of reinternal organization in `base` that was needed for this to
happen:

* `Data.Foldable`, `Data.Traversable`, `Data.Bifoldable`, and
  `Data.Bitraversable` share some nonexported datatypes (e.g., `StateL`,
  `StateR`, `Min`, `Max`, etc.) to implement some instances. To avoid
  code duplication, I migrated this internal code to a new hidden
  module, `Data.Functor.Utils` (better naming suggestions welcome).

* `Data.Traversable` and `Data.Bitraversable` also make use of an
  identity newtype, so I modified them to use
  `Data.Functor.Identity.Identity`. This has a ripple effect on several
  other modules, since I had to move instances around in order to avoid
  dependency cycles.

Fixes #10448.

Reviewers: ekmett, hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2284

GHC Trac Issues: #9682, #10448