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.