Add interface file for Rpc_std_helpers module

This is cherry-picked from 8f970ce

Signed-off-by: Gabor Igloi <[email protected]>

Author: gaborigloi

ocaml/xapi/rpc_std_helpers.ml

@@ -1,5 +1,3 @@
-(** Helpers to marshal and unmarshal Rpc.Dict into Hashtbl *)
-
 let rpc_of_hashtbl ~rpc_of t =
   let dict = Hashtbl.fold (fun k v acc -> (k, rpc_of v) :: acc) t [] in
   Rpc.Dict dict

ocaml/xapi/rpc_std_helpers.mli

@@ -0,0 +1,5 @@
+(** Helpers to marshal and unmarshal Rpc.Dict into Hashtbl *)
+
+val rpc_of_hashtbl : rpc_of:('a -> Rpc.t) -> (string, 'a) Hashtbl.t -> Rpc.t
+
+val hashtbl_of_rpc : of_rpc:(Rpc.t -> 'a) -> Rpc.t -> (string, 'a) Hashtbl.t