Review all the *Client receiver methods in layer3.go. They should
probably either all be receivers, or none.

Maybe put auth-related stuff into its own structure inside Client,
instead of at Client's top level.

Add a way to broadcast status information as negotiation happens or
disconnects occur. Possibly a new type of object that can be sent on
Recv along with stanzas. Or use sync.Cond to protect a state
variable.

Asynchronously updating Client.Features is not thread safe. Use a
public function GetFeatures() or similar. Same for setting the JID in
bind()?

NewClient should be synchronous, so it can update Features and JID and
leave those as values rather than functions.

Add a Reconnect() function.

Put roster and bind into separate packages, if possible.

Eliminate as many uses of Generic as possible.

Callback doesn't need to return bool. It shouldn't affect what's given
to the client.