Refactor occurrence-check logic
This patch does two related things * Combines the occurrence-check logic in the on-the-fly unifier with that in the constraint solver. They are both doing the same job, after all. The resulting code is now in TcUnify: metaTyVarUpdateOK occCheckExpand occCheckForErrors (called in TcErrors) * In doing this I disovered checking for family-free-ness and foralls can be unnecessarily inefficient, because it expands type synonyms. It's easy just to cache this info in the type syononym TyCon, which I am now doing.
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