Better solving for representational equalities
This patch adds a bit of extra solving power for representational equality constraints to fix Trac #14333 The main changes: * Fix a buglet in TcType.isInsolubleOccursCheck which wrongly reported a definite occurs-check error for (a ~R# b a) * Get rid of TcSMonad.emitInsolubles. It had an ad-hoc duplicate-removal piece that is better handled in interactIrred, now that insolubles are Irreds. We need a little care to keep inert_count (which does not include insolubles) accurate. * Refactor TcInteract.solveOneFromTheOther, to return a much simpler type. It was just over-complicated before. * Make TcInteract.interactIrred look for constraints that match either way around, in TcInteract.findMatchingIrreds This wasn't hard and it cleaned up quite a bit of code.
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