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.

NewClient shouldn't be asynchronous.

Add a Reconnect() function.

Put roster and bind into separate packages, if possible.

Eliminate as many uses of Generic as possible.