xmpp-parsers: Convert Forwarded to xso

parsers/ChangeLog

@@ -19,6 +19,11 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
         check whether a child element is present or not, as this is currently
         implemented in xso.  We might revert that change if we implement flag
         metas in xso before the next release.
+      - The forwarding module now hardcodes that its child is a message in the
+        jabber:client namespace.  It previously half-followed the schema, but
+        nothing so far uses it for anything but this message.  We can always
+        change it again once any other specification allows e.g. presences or
+        iqs.
     * New parsers/serialisers:
         - Stream Features (RFC 6120) (!400)
         - Extensible SASL Profile (XEP-0388)