tokio_xmpp: properly process <stream:error/> while waiting for features

Before this, a stream error would not be readable by user code, as the
`recv_features()` function would not even attempt to parse it. This
change allows application code to react to stream errors which are
received before stream features are received.

skip-changelog, because there's no release with the xmlstream module
yet.

parsers/src/iq.rs

@@ -39,6 +39,15 @@ impl IqHeader {
     }
 }
 
+/// Payload of an IQ request stanza.
+pub enum IqRequestPayload {
+    /// Payload of a type='get' stanza.
+    Get(Element),
+
+    /// Payload of a type='set' stanza.
+    Set(Element),
+}
+
 /// Payload of an IQ stanza, by type.
 pub enum IqPayload {
     /// Payload of a type='get' stanza.