xmpp-parsers: Split ibr::Query into multiple structs

This introduces FieldsQuery, RemoveQuery, LegacyQuery and FormsQuery,
making this module reflect more accurately how it gets used on XMPP.

parsers/ChangeLog

@@ -4,6 +4,9 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
         - The type of jingle_rtp::Description::ssrc has been changed from
           Option<String> to Option<u32>, in accordance with XEP-0167 version
           1.2.3 and RFC 3550. (!602)
+        - The ibr::Query struct has been split into FieldsQuery, RemoveQuery,
+          LegacyQuery and FormsQuery, to make it reflect more how it gets
+          used in XMPP.
     * Improvements:
         - Make Priority’s inner i8 pub, which had been broken since the
           conversion to xso. (!632)