Arity 0 is not only for int.

Patch from @IwanKaramazow

Author: cristianoc

src/res_outcome_printer.ml

@@ -239,12 +239,12 @@ let printPolyVarIdent txt =
        ]
      | Otyp_constr (
         Oide_dot (Oide_dot (Oide_ident "Js", "Fn") , "arity0"), (* Js.Fn.arity0 *)
-        [Otyp_constr (Oide_ident ident, [])] (* int or unit or string *)
+        [typ] 
        ) ->
-        (* Js.Fn.arity0<int> -> (.) => int*)
+        (* Js.Fn.arity0<t> -> (.) => t *)
         Doc.concat [
           Doc.text "(.) => ";
-          Doc.text ident;
+          printOutTypeDoc typ;
         ]
      | Otyp_constr (
         Oide_dot (Oide_dot (Oide_ident "Js", "Fn") , ident), (* Js.Fn.arity2 *)

tests/oprint/expected/oprint.resi.txt

@@ -179,8 +179,8 @@ module Js: {
 }
 type arity0 = (.) => unit
 type arity0b = (.) => int
-type arity0c = Js.Fn.arity0<Js.Fn.arity0<array<int>>>
-type arity0d = (. unit) => unit
+type arity0c = (.) => (.) => array<int>
+type arity0d = (.) => unit => unit
 type arity1 = (. int) => int
 type arity2 = (. int, int) => int
 type arity3 = (. int, int, int) => int