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

src/message_correct.rs

@@ -4,6 +4,8 @@
 // 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_element!(
     /// Defines that the message containing this payload should replace a
     /// previous message, identified by the id.
@@ -14,6 +16,8 @@ generate_element!(
     ]
 );
 
+impl MessagePayload for Replace {}
+
 #[cfg(test)]
 mod tests {
     use super::*;