Note [TyVarTv]

A TyVarTv can unify with type *variables* only, including other TyVarTvs and
skolems.  They are used in two places:

1. In kind signatures, see GHC.Tc.TyCl
      Note [Inferring kinds for type declarations]
   and Note [Using TyVarTvs for kind-checking GADTs]

2. In partial type signatures.  See GHC.Tc.Types
   Note [Quantified variables in partial type signatures]

Sometimes, they can unify with type variables that the user would
rather keep distinct; see #11203 for an example.  So, any client of
this function needs to either allow the TyVarTvs to unify with each
other or check that they don't. In the case of (1) the check is done
in GHC.Tc.TyCl.swizzleTcTyConBndrs.  In case of (2) it's done by
findDupTyVarTvs in GHC.Tc.Gen.Bind.chooseInferredQuantifiers.

Historical note: Before #15050 this (under the name SigTv) was also
used for ScopedTypeVariables in patterns, to make sure these type
variables only refer to other type variables, but this restriction was
dropped, and ScopedTypeVariables can now refer to full types (GHC
Proposal 29).