Rename SigTv to TyVarTv (Trac #15480)
because since Trac #15050, these are no longer used in pattern SIGnatures,
but still in other places where meta-variables should only be unified
with TYpe VARiables.
I also found mentions of SigTv in parts of the renamer and desugarer
that do not seem to directly relate to SigTv as used in the type
checker, but rather to uses of forall a. in type signatures. I renamed
these to ScopedTv.
Differential Revision: https://phabricator.haskell.org/D5074