Refactor functions related to coercion optimisation?

There are quite a few functions that (to the best of my knowledge) are used only for coercion optimisation. In particular I think this holds for the following in GHC.Core.Coercion:

setNominalRole_maybe
splitAppCo_maybe
promoteCoercion
instCoercion
instCoercions

There's also liftCoMatch and the functions it calls in GHC.Core.Unify.

Would it make sense to factor these out into a separate module that is imported only from GHC.Core.Coercion.Opt? That would seem better from a modularity perspective and make it clear they aren't intended for "general" use.

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