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

src/attention.rs

@@ -4,11 +4,15 @@
 // 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 the attention of the recipient.
     Attention, "attention", ATTENTION
 );
 
+impl MessagePayload for Attention {}
+
 #[cfg(test)]
 mod tests {
     use super::*;