This lets us kill fsEnvElts function which is nondeterministic.
We also get better guarantees than just comments.
We don't do lookups, but I believe a set is needed for deduplication.

Test Plan: ./validate

Reviewers: bgamari, mpickering, austin, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2297

GHC Trac Issues: #4012