add amount.mli

src/amount.mli

@@ -0,0 +1,21 @@
+type sign =
+  | Sign_plus
+  | Sign_minus
+
+type t = private {
+  sign: sign option;
+  currency: string;
+  value: Int64.t;
+  fraction: Int32.t;
+}
+
+val make :
+  sign:sign option ->
+  currency:string ->
+  value:Int64.t ->
+  fraction:Int32.t ->
+  (t, string) result
+
+val pp : Format.formatter -> t -> unit
+val to_string : t -> string
+val of_string : string -> (t, string) result