Improve piResultTys and friends
Several things here: * Re-implement piResultTys so that its substitution has the correct in-scope set That means paying close attention to performance, since as we discovered in Trac #11371, it's a heavily used function and is often used on ordinary function types, with no foralls to worry about substituting. * Kill off applyTys, which was just the same as piResultTys. * Re-engineer MkCore.mkCoreApps so that it calls piResultTys, rather than repeatedly calling piResultTy.
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