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

src/caps.rs

@@ -6,6 +6,7 @@
 
 use try_from::TryFrom;
 
+use presence::PresencePayload;
 use disco::{Feature, Identity, DiscoInfoResult, DiscoInfoQuery};
 use data_forms::DataForm;
 use hashes::{Hash, Algo};
@@ -39,6 +40,8 @@ pub struct Caps {
     pub hash: Hash,
 }
 
+impl PresencePayload for Caps {}
+
 impl TryFrom<Element> for Caps {
     type Err = Error;