message: Make MessagePayload a trait, and implement it on every payload.

src/receipts.rs

@@ -4,12 +4,16 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+use message::MessagePayload;
+
 generate_empty_element!(
     /// Requests that this message is acked by the final recipient once
     /// received.
     Request, "request", RECEIPTS
 );
 
+impl MessagePayload for Request {}
+
 generate_element!(
     /// Notes that a previous message has correctly been received, it is
     /// referenced by its 'id' attribute.
@@ -20,6 +24,8 @@ generate_element!(
     ]
 );
 
+impl MessagePayload for Received {}
+
 #[cfg(test)]
 mod tests {
     use super::*;