Note [No Ord for Unique]

As explained in Note [Unique Determinism] the relative order of Uniques
is nondeterministic. To prevent from accidental use the Ord Unique
instance has been removed.
This makes it easier to maintain deterministic builds, but comes with some
drawbacks.
The biggest drawback is that Maps keyed by Uniques can't directly be used.
The alternatives are:

  1) Use UniqFM or UniqDFM, see Note [Deterministic UniqFM] to decide which
  2) Create a newtype wrapper based on Unique ordering where nondeterminism
     is controlled. See GHC.Unit.Module.Env.ModuleEnv
  3) Change the algorithm to use nonDetCmpUnique and document why it's still
     deterministic
  4) Use TrieMap as done in GHC.Cmm.CommonBlockElim.groupByLabel