Optimise `Identity` instances with `coerce`

This also overrides all optional `Foldable` methods
(which would otherwise be default-implemented in terms of `foldMap`)
with supposedly optimally minimal implementations.

While at it, this also removes the redundant `{-# LANGUAGE CPP #-}`.

Reviewed By: austin, dfeuer

Differential Revision: https://phabricator.haskell.org/D467