Ryan Scott authored Nov 13, 2020 and Marge Bot committed Nov 15, 2020

There is a zoo of `splitForAll-` functions in `GHC.Core.Type` (as well as
`tcSplitForAll-` functions in `GHC.Tc.Utils.TcType`) that all do very similar
things, but vary in the particular form of type variable that they return. To
make things worse, the names of these functions are often quite misleading.
Some particularly egregious examples:

* `splitForAllTys` returns `TyCoVar`s, but `splitSomeForAllTys` returns
  `VarBndr`s.
* `splitSomeForAllTys` returns `VarBndr`s, but `tcSplitSomeForAllTys` returns
  `TyVar`s.
* `splitForAllTys` returns `TyCoVar`s, but `splitForAllTysInvis` returns
  `InvisTVBinder`s. (This in particular arose in the context of #18939, and
  this finally motivated me to bite the bullet and improve the status quo
  vis-à-vis how we name these functions.)

In an attempt to bring some sanity to how these functions are named, I have
opted to rename most of these functions en masse to use consistent suffixes
that describe the particular f...