Note [When to print foralls]

We opt to explicitly pretty-print `forall`s if any of the following
criteria are met:

1. -fprint-explicit-foralls is on.

2. A bound type variable has a polymorphic kind. E.g.,

     forall k (a::k). Proxy a -> Proxy a

   Since a's kind mentions a variable k, we print the foralls.

3. A bound type variable is a visible argument (#14238).
   Suppose we are printing the kind of:

     T :: forall k -> k -> Type

   The "forall k ->" notation means that this kind argument is required.
   That is, it must be supplied at uses of T. E.g.,

     f :: T (Type->Type)  Monad -> Int

   So we print an explicit "T :: forall k -> k -> Type",
   because omitting it and printing "T :: k -> Type" would be
   utterly misleading.

   See Note [VarBndrs, ForAllTyBinders, TyConBinders, and visibility]
   in GHC.Core.TyCo.Rep.

N.B. Until now (Aug 2018) we didn't check anything for coercion variables.