Note [Promote monomorphic tyvars]

Promote any type variables that are free in the environment.  Eg
   f :: forall qtvs. bound_theta => zonked_tau
The free vars of f's type become free in the envt, and hence will show
up whenever 'f' is called.  They may currently at rhs_tclvl, but they
had better be unifiable at the outer_tclvl!  Example: envt mentions
alpha[1]
           tau_ty = beta[2] -> beta[2]
           constraints = alpha ~ [beta]
we don't quantify over beta (since it is fixed by envt)
so we must promote it!  The inferred type is just
  f :: beta -> beta

NB: promoteTyVarSet ignores coercion variables