The `Ord Module` instance currently uses `Unique`s for comparison.
We don't want to use the `Unique` order because it can introduce nondeterminism.
This switches `Ord ModuleName` and `Ord UnitId` to use lexicographic ordering
making `Ord Module` deterministic transitively.

I've run `nofib` and it doesn't make a measurable difference.

See also Note [ModuleEnv determinism and performance].

Test Plan:
./validate
run nofib: P112

Reviewers: simonpj, simonmar, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2030

GHC Trac Issues: #4012