We want to remove the `Ord Unique` instance because there's
no way to implement it in deterministic way and it's too
easy to use by accident.

We sometimes compute SCC for datatypes whose Ord instance
is implemented in terms of Unique. The Ord constraint on
SCC is just an artifact of some internal data structures.
We can have an alternative implementation with a data
structure that uses Uniquable instead.

This does exactly that and I'm pleased that I didn't have
to introduce any duplication to do that.

Test Plan:
./validate
I looked at performance tests and it's a tiny bit better.

Reviewers: bgamari, simonmar, ezyang, austin, goldfire

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2359

GHC Trac Issues: #4012