This accomplishes three things:

* Adds missing `Alternative`, `MonadPlus`, and `MonadZip` instances for
  `Proxy`
* Adds a missing `MonadPlus` instance for `U1`
* Changes several existing `U1` instances to use lazy pattern-matching,
  exactly how `Proxy` does it (in case we ever replace `U1` with
  `Proxy`). This is technically a breaking change (albeit an extremely
  minor one).

Test Plan: ./validate

Reviewers: austin, ekmett, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1962

GHC Trac Issues: #11650