This patch was provoked by Trac #14363.  Turned out that we were
kicking out too many constraints in TcSMonad.kickOutRewritable, and
that mean that the work-list never became empty: infinite loop!

That in turn made me look harder at the Main Theorem in
Note [Extending the inert equalities].

Main changes

* Replace TcType.isTyVarExposed by TcType.isTyVarHead.  The
  over-agressive isTyVarExposed is what caused Trac #14363.
  See Note [K3: completeness of solving] in TcSMonad.

* TcType.Make anyRewriteableTyVar role-aware.  In particular,
      a ~R ty
  cannot rewrite
      b ~R f a
  See Note [anyRewriteableTyVar must be role-aware].  That means
  it has to be given a role argument, which forces a little
  refactoring.

  I think this change is fixing a bug that hasn't yet been reported.
  The actual reported bug is handled by the previous bullet.  But
  this change is definitely the Right Thing

The main changes are in TcSMonad.kick_out_rewritable, and in TcType
(isTyVarExposed ---> isTyVarHead).

I did a little unforced refactoring:

 * Use the cc_eq_rel field of a CTyEqCan when it is available, rather
   than recomputing it.

 * Define eqCanRewrite :: EqRel -> EqRel -> EqRel, and use it, instead
   of duplicating its logic