presence: Make PresencePayload a trait, and implement it on every payload.

src/ecaps2.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 presence::PresencePayload;
 use disco::{Feature, Identity, DiscoInfoResult, DiscoInfoQuery};
 use data_forms::DataForm;
 use hashes::{Hash, Algo};
@@ -27,6 +28,8 @@ generate_element!(
     ]
 );
 
+impl PresencePayload for ECaps2 {}
+
 fn compute_item(field: &str) -> Vec<u8> {
     let mut bytes = field.as_bytes().to_vec();
     bytes.push(0x1f);