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This MR speeds up type equality, triggered by perf regressions that showed up when fixing #24725 by parameterising type equality over whether to ignore multiplicity. The changes are: * Do not use `nonDetCmpType` for type /equality/. Instead use a specialised type-equality function, which we have always had! `nonDetCmpType` remains, but I did not invest effort in refactoring or optimising it. * Type equality is parameterised by - whether to expand synonyms - whether to respect multiplicities - whether it has a RnEnv2 environment In this MR I systematically specialise it for static values of these parameters. Much more direct and predictable than before. See Note [Specialising type equality] * We want to avoid comparing kinds if possible. I refactored how this happens, at least for `eqType`. See Note [Casts and coercions in type comparison] * To make Lint fast, we want to avoid allocating a thunk for <msg> in ensureEqTypes ty1 ty2 <msg> becau...
