Fixed unique token generation in genfft

Unique token generation was based on the assumption that OCaml compiler
won't inline a given piece of code. Starting from 4.02.0 it does more
aggressive inlining and breaks this functionality.

genfft/unique.ml

@@ -21,18 +21,14 @@
 
 (* repository of unique tokens *)
 
-type unique = Unique of unit
+type unique = int
 
-(* this depends on the compiler not being too smart *)
-let make () =
-  let make_aux x = Unique x in
-  make_aux ()
-
-(* note that the obvious definition
+let id = ref 0
 
-      let make () = Unique ()
+let make () =
+  id := !id + 1;
+  !id
 
-   fails *)
+let same a b =
+  a = b
 
-let same (a : unique) (b : unique) =
-  (a == b)