Note [Variable Specificity and Forall Visibility]

A HsForAllTy contains an HsForAllTelescope to denote the visibility of the forall
binder. Furthermore, each invisible type variable binder also has a
Specificity. Together, these determine the variable binders (ForAllTyFlag) for each
variable in the generated ForAllTy type.

This table summarises this relation:

| User-written type         HsForAllTelescope   Specificity        ForAllTyFlag
|---------------------------------------------------------------------------
| f :: forall a. type       HsForAllInvis       SpecifiedSpec      Specified
| f :: forall {a}. type     HsForAllInvis       InferredSpec       Inferred
| f :: forall a -> type     HsForAllVis         SpecifiedSpec      Required
| f :: forall {a} -> type   HsForAllVis         InferredSpec       /
|   This last form is nonsensical and is thus rejected.


For more information regarding the interpretation of the resulting ForAllTyFlag, see
Note [VarBndrs, ForAllTyBinders, TyConBinders, and visibility] in "GHC.Core.TyCo.Rep".