xmpp-parsers: Convert vCard-temp to xso

This also keeps all additional elements in the vCard, so they get
serialized back on the wire instead of being dropped.

parsers/ChangeLog

@@ -28,6 +28,10 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
       - The bookmarks2 Conference `extensions` child is now an
         `Option<Extensions>` instead of a `Vec<Element>`, to distinguish
         between it being absent or empty (!472).
+    * Improvements:
+      - Keep unsupported vCard elements as `minidom::Element`, so that they
+        get serialized back instead of being dropped.  We now also test for
+        the size of these elements (!472).
     * New parsers/serialisers:
         - Stream Features (RFC 6120) (!400)
         - Extensible SASL Profile (XEP-0388)