Refactor skolemising, and newClsInst
This makes newClsInst (was mkInstance) look more like newFamInst, and simplifies the plumbing of the overlap flag, and ensures that freshening (required by the InstEnv stuff) happens in one place. On the way I also tided up the rather ragged family of tcInstSkolTyVars and friends. The result at least has more uniform naming.
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