For some time we have had two free-variable mechanims for types:

* The "FV" mechanism, embodied in GHC.Utils.FV, which worked OK, but
  was fragile where eta-expansion was concerned.

* The TyCoFolder mechanism, using a one-shot EndoOS accumulator

I finally got tired of this and refactored the whole thing, thereby
addressing #27080.   Now we have

* `GHC.Types.Var.FV`, which has a composable free-variable result type,
  very much in the spirit of the old `FV`, but much more robust.
  (It uses the "one shot trick".)

* GHC.Core.TyCo.FVs now has just one technology for free variables.

All this led to a lot of renaming.

There are couple of error-message changes.  The change in T18451
makes an already-poor error message even more mysterious.  But
it really needs a separate look.

We also now traverse the AST in a different order leading to a different
but still deterministic order for FVs and test output has been adjusted
accordingly.