Added a utility function to extract the udp handle from udp send requests.

src/rt/rust_uv.cpp

@@ -330,6 +330,11 @@ rust_uv_udp_recv_stop(uv_udp_t* server) {
     return uv_udp_recv_stop(server);
 }
 
+extern "C" uv_udp_t*
+rust_uv_get_udp_handle_from_send_req(uv_udp_send_t* send_req) {
+    return send_req->handle;
+}
+
 extern "C" int
 rust_uv_listen(uv_stream_t* stream, int backlog,
         uv_connection_cb cb) {

src/rt/rustrt.def.in

@@ -111,6 +111,7 @@ rust_uv_udp_send
 rust_uv_udp_send6
 rust_uv_udp_recv_start
 rust_uv_udp_recv_stop
+rust_uv_get_udp_handle_from_send_req
 rust_uv_listen
 rust_uv_accept
 rust_uv_write