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This implements `DVarSet`, a deterministic set of Vars, with an interface very similar to `VarSet` with a couple of functions missing. I will need this in changes that follow, one of them will be about changing the type of the set of Vars that `RuleInfo` holds to make the free variable computation deterministic. Test Plan: ./validate I can add new tests if anyone wants me to. Reviewers: simonpj, simonmar, austin, bgamari Reviewed By: simonmar, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1487 GHC Trac Issues: #4012
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