As a result of some other unrelated changes I found that
IndTypesPerf was failing, and opened Trac #11408.  There's
a test in indexed-types/should-compile/T11408.

The bug was that a type like
 forall t. (MT (UL t) (UR t) ~ t) => UL t -> UR t -> Int
is in fact unambiguous, but it's a bit subtle to prove
that it is unambiguous.

In investigating, Dimitrios and I found several subtle
bugs in the constraint solver, fixed by this patch

* canRewrite was missing a Derived/Derived case.  This was
  lost by accident in Richard's big kind-equality patch.

* Interact.interactTyVarEq would discard [D] a ~ ty if there
  was a [W] a ~ ty in the inert set.  But that is wrong because
  the former can rewrite things that the latter cannot.
  Fix: a new function eqCanDischarge

* In TcSMonad.addInertEq, the process was outright wrong for
  a Given/Wanted in the (GWModel) case.  We were adding a new
  Derived without kicking out things that it could rewrite.
  Now the code is simpler (no special GWModel case), and works
  correctly.

* The special case in kickOutRewritable for [W] fsk ~ ty,
  turns out not to be needed.  (We emit a [D] fsk ~ ty which
  will do the job.

I improved comments and documentation, esp in TcSMonad.