Note [quantifyTyVars determinism]

The results of quantifyTyVars are wrapped in a forall and can end up in the
interface file. One such example is inferred type signatures. They also affect
the results of optimizations, for example worker-wrapper. This means that to
get deterministic builds quantifyTyVars needs to be deterministic.

To achieve this CandidatesQTvs is backed by deterministic sets which allows them
to be later converted to a list in a deterministic order.

For more information about deterministic sets see
Note [Deterministic UniqFM] in GHC.Types.Unique.DFM.