xmpp-parsers: Make DiscoInfoResult::features a BTreeSet

Features must never be duplicated, so we can use a BTreeSet instead of a
Vec to be nicer for users.

It makes the internal API for computing caps and ecaps2 a bit worse,
because it was expecting a slice directly, so for now let’s collect the
BTreeSet into a Vec when computing the thing.  A refactor to use
Iterator might make it better eventually, but I won’t work on that
before profiling it.

parsers/src/disco.rs

@@ -10,6 +10,7 @@ use crate::data_forms::DataForm;
 use crate::iq::{IqGetPayload, IqResultPayload};
 use crate::ns;
 use crate::rsm::{SetQuery, SetResult};
+use alloc::collections::BTreeSet;
 use jid::Jid;
 
 /// Structure representing a `<query xmlns='http://jabber.org/protocol/disco#info'/>` element.
@@ -98,7 +99,7 @@ pub struct DiscoInfoResult {
 
     /// List of features supported by this entity.
     #[xml(extract(n = .., name = "feature", fields(attribute(name = "var", type_ = String))))]
-    pub features: Vec<String>,
+    pub features: BTreeSet<String>,
 
     /// List of extensions reported by this entity.
     #[xml(child(n = ..))]