Note [Printing foralls in type family instances]

We use the same criteria as in Note [When to print foralls] to determine
whether a type family instance should be pretty-printed with an explicit
`forall`. Example:

  type family Foo (a :: k) :: k where
    Foo Maybe       = []
    Foo (a :: Type) = Int
    Foo a           = a

Without -fprint-explicit-foralls enabled, this will be pretty-printed as:

type family Foo (a :: k) :: k where
  Foo Maybe = []
  Foo a = Int
  forall k (a :: k). Foo a = a

Note that only the third equation has an explicit forall, since it has a type
variable with a non-Type kind. (If -fprint-explicit-foralls were enabled, then
the second equation would be preceded with `forall a.`.)

There is one tricky point in the implementation: what visibility
do we give the type variables in a type family instance? Type family instances
only store type *variables*, not type variable *binders*, and only the latter
has visibility information. We opt to default the visibility of each of these
type variables to Specified because users can't ever instantiate these
variables manually, so the choice of visibility is only relevant to
pretty-printing. (This is why the `k` in `forall k (a :: k). ...` above is
printed the way it is, even though it wasn't written explicitly in the
original source code.)

We adopt the same strategy for data family instances. Example:

  data family DF (a :: k)
  data instance DF '[a, b] = DFList

That data family instance is pretty-printed as:

  data instance forall j (a :: j) (b :: j). DF '[a, b] = DFList

This is despite that the representation tycon for this data instance (call it
$DF:List) actually has different visibilities for its binders.
However, the visibilities of these binders are utterly irrelevant to the
programmer, who cares only about the specificity of variables in `DF`'s type,
not $DF:List's type. Therefore, we opt to pretty-print all variables in data
family instances as Specified.