Export `Traversable()` and `Foldable()` from Prelude

This exposes *only* the type-classes w/o any of their methods.
This is the very first step for implementing BPP (see #9586), which
already requires breaking up several import-cycles leading back to `Prelude`.

Ideally, importing `Prelude` should be avoided in most `base` modules,
as `Prelude` does not define any entities, but rather re-exports
existing ones.

Test Plan: validate passes

Reviewers: ekmett, austin

Reviewed By: ekmett, austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D209

GHC Trac Issues: #9586