Note [Required foralls in Core]

Consider the CoreExpr (Lam a e) where `a` is a TyVar, and (e::e_ty).
It has type
   forall a. e_ty
Note the Specified visibility of (forall a. e_ty); the Core type just isn't able
to express more than one visiblity, and we pick `Specified`.  See `exprType` and
`mkLamType` in GHC.Core.Utils, and `GHC.Type.Var.coreTyLamForAllTyFlag`.

So how can we ever get a term of type (forall a -> e_ty)?  Answer: /only/ via a
cast built with ForAllCo.  See `GHC.Core.Coercion.mkForAllVisCos`,
`GHC.Tc.Types.Evidence.mkWpForAllCast` and `GHC.Core.Make.mkCoreTyLams`.
This does not seem very satisfying, but it does the job.

An alternative would be to put a visibility flag into `Lam` (a huge change),
or into a `TyVar` (a more plausible change), but we leave that for the future.

See also Note [ForAllTy and type equality] in GHC.Core.TyCo.Compare.