Make Set.to_array use Enum instead of Dynarray

This breaks the dependency Set -> Dynarray -> Int. A quick benchmark
also shows that the new function has comparable performances and can
avoid Out_of_memory.

src/batSet.ml

@@ -331,11 +331,6 @@ module Concrete = struct
   let elements s = elements_aux [] s
   let to_list = elements
 
-  let to_array s =
-    let acc = BatDynArray.create () in
-    iter (BatDynArray.add acc) s;
-    BatDynArray.to_array acc
-
   let rec cons_iter s t = match s with
       Empty -> t
     | Node (l, e, r, _) -> cons_iter l (C (e, r, t))
@@ -365,6 +360,9 @@ module Concrete = struct
       BatEnum.make ~next:(enum_next l) ~count:(enum_count l) ~clone
     in make (cons_iter t E)
 
+  let to_array s =
+    BatArray.of_enum (enum s)
+
   let backwards t =
     let rec make l =
       let l = ref l in