Equality of forall-types is visibility aware

This patch finally (I hope) nails the question of whether
   (forall a. ty) and (forall a -> ty)
are `eqType`: they aren't!

There is a long discussion in #22762, plus useful Notes:

* Note [ForAllTy and type equality] in GHC.Core.TyCo.Compare
* Note [Comparing visiblities] in GHC.Core.TyCo.Compare
* Note [ForAllCo] in GHC.Core.TyCo.Rep

It also establishes a helpful new invariant for ForAllCo,
and ForAllTy, when the bound variable is a CoVar:in that
case the visibility must be coreTyLamForAllTyFlag.

All this is well documented in revised Notes.