Note [Solve by unification]

If we solve
   alpha[n] ~ ty
by unification, there are two cases to consider

* TouchableSameLevel: if the ambient level is 'n', then
  we can simply update alpha := ty, and do nothing else

* TouchableOuterLevel free_metas n: if the ambient level is greater than
  'n' (the level of alpha), in addition to setting alpha := ty we must
  do two other things:

  1. Promote all the free meta-vars of 'ty' to level n.  After all,
     alpha[n] is at level n, and so if we set, say,
          alpha[n] := Maybe beta[m],
     we must ensure that when unifying beta we do skolem-escape checks
     etc relevant to level n.  Simple way to do that: promote beta to
     level n.

  2. Set the Unification Level Flag to record that a level-n unification has
     taken place. See Note [The Unification Level Flag] in GHC.Tc.Solver.Monad

NB: TouchableSameLevel is just an optimisation for TouchableOuterLevel. Promotion
would be a no-op, and setting the unification flag unnecessarily would just
make the solver iterate more often.  (We don't need to iterate when unifying
at the ambient level because of the kick-out mechanism.)