Properly normalise reduced dicts

Properly normalise reduced dicts
- Another chapter in the never-ending TcSimplify.reduceContext saga: after
  context reduction of wanted dicts it is not sufficient to normalise them
  wrt to the wanted equalities.  We also need to take top-level equalities
  into account.  (In fact, we probably also have to normalise wrt to given
  equalities, but I have left that open for the moment - but added a TODO
  note.)
- This finally eliminates substEqInDictInsts from TcTyFuns interface and
  suggest some further possible clean up (which will be in a separate patch).

Thanks to Roman for the intricate example that uncovered this bug.