Fix a number of subtle solver bugs
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.