Type splayed recursive types cannot have parameters

[brandonzstride]

When type splaying, generation of recursive types is stubbed with an unusable value. However, in the presence of parametric recursive types, the mu type is actually a type function, and this encoding fails.

For example:

let t = mu t a.
  | `Nil of unit
  | `Cons of { hd : a ; tl : t a }

let rec length (type a) (ls : t a) : int =
  match ls with
  | `Nil _ -> 0
  | `Cons r -> 1 + length a r.tl
  end

We get application to a non-function.

The simple fix should be to let recursive type stubs accept and hold onto parameters.