Use UniqFM for SigOf
Use UniqFM for SigOf

Summary:
The Ord instance for ModuleName is currently implemented in
terms of Uniques causing potential determinism problems.
I plan to change it to use the actual FastStrings and in
preparation for that I'm switching to UniqFM where it's
possible (you need *one* Unique per key, and you can't get
the keys back), so that the performance doesn't suffer.

Test Plan: ./validate

Reviewers: simonmar, austin, ezyang, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2320

GHC Trac Issues: #4012

(cherry picked from commit 586d55815401c54f4687d053fb033e53865e0bf1)

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