Performance enhancements in TcFlatten.
This commit fixes some performance regressions introduced by 0cc47eb9, adding more `Coercible` magic to the solver. See Note [flatten_many performance] in TcFlatten for more info. The improvements do not quite restore the old numbers. Given that the solver is really more involved now, I am accepting this regression. The way forward (I believe) would be to have *two* flatteners: one that deals only with nominal equalities and thus never checks roles, and the more general one. A nice design of keeping this performant without duplicating code eludes me, but someone else is welcome to take a stab.
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