add amount

test/test.ml

@@ -18,3 +18,18 @@ let () =
   check_bad Timestamp.jsont {|{"t_s": "123456780"}|};
   check_bad Timestamp.jsont {|{"t_s": "agagou"}|};
   ()
+
+let () =
+  let open Types.Amount in
+  let v =
+    {
+      sign= Some `Plus
+    ; currency= `Eur
+    ; value= Int64.of_int 25
+    ; fraction= Int64.of_int 678
+    }
+  in
+  let s = to_string v in
+  let v' = of_string s |> Result.get_ok in
+  assert (v = v');
+  ()