parsers: improve xml:lang handling

This is still far from being a full fix. Still, we can avoid the lang
attribute (by allowing discarding the `xml:lang` value via the feature
introduced a couple commits ago) and we can avoid emitting `xml:lang=""`
where no language was previously set using the mechanism introduced for
<message/> in 5172fb5e.

parsers/ChangeLog

@@ -61,7 +61,6 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
         Handshake::from_stream_id_and_password(), with the two parameters
         having been exchanged, and the stream_id is now a String instead of
         &str.
-      - Add a lang property in Presence, to avoid parser errors.
       - pubsub::Owner is now a wrapper for the pubsub::owner::Paylolad enum,
         and all its direct children have been merged into this enum (!532)
       - time::TimeResult has been ported to use xso. Use From/Into to convert
@@ -84,6 +83,8 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
         3. the message::Thread type was converted from a single-element
            tuple-like struct to a struct with named fields to support the
            optional `parent` attribute.
+      - presence::Presence has been ported to use xso (!477). It also uses the
+        message::Lang newtype since !561.
     * New parsers/serialisers:
       - Stream Features (RFC 6120) (!400)
       - Spam Reporting (XEP-0377) (!506)