Implement sscanf in terms of ksscanf

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

src/stdune/scanf.ml

@@ -1,12 +1,14 @@
 module Scanf = Dune_caml.Scanf
 
-let try_ f =
-  match f () with
-  | exception End_of_file -> Error ()
+let unescaped x =
+  match Scanf.unescaped x with
   | exception Scanf.Scan_failure _ -> Error ()
-  | exception (Failure _) -> Error ()
   | x -> Ok x
 
-let unescaped x = try_ (fun () -> Scanf.unescaped x)
-
-let sscanf x fmt f = try_ (fun () -> Scanf.sscanf x fmt f)
+let sscanf x fmt f =
+  let module M = struct
+    exception E
+  end in
+  match Scanf.ksscanf x (fun _ _ -> raise_notrace M.E) fmt f with
+  | exception M.E -> Error ()
+  | x -> Ok x