Note [CandidatesQTvs determinism and order]

* Determinism: when we quantify over type variables we decide the
  order in which they appear in the final type. Because the order of
  type variables in the type can end up in the interface file and
  affects some optimizations like worker-wrapper, we want this order to
  be deterministic.

  To achieve that we use deterministic sets of variables that can be
  converted to lists in a deterministic order. For more information
  about deterministic sets see Note [Deterministic UniqFM] in GHC.Types.Unique.DFM.

* Order: as well as being deterministic, we use an
  accumulating-parameter style for candidateQTyVarsOfType so that we
  add variables one at a time, left to right.  That means we tend to
  produce the variables in left-to-right order.  This is just to make
  it bit more predictable for the programmer.