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

src/stanza_id.rs

@@ -4,6 +4,7 @@
 // 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;
 use jid::Jid;
 
 generate_element!(
@@ -19,6 +20,8 @@ generate_element!(
     ]
 );
 
+impl MessagePayload for StanzaId {}
+
 generate_element!(
     /// A hack for MUC before version 1.31 to track a message which may have
     /// its 'id' attribute changed.
@@ -29,6 +32,8 @@ generate_element!(
     ]
 );
 
+impl MessagePayload for OriginId {}
+
 #[cfg(test)]
 mod tests {
     use super::*;