Extension.StanzaHandlers should use reflection, not constructor
functions.

Rather than use Client.AddFilter(), and Extension.Start(), we should
have a function in Extension that, if non-nil, accepts a stanza and
returns a slice of stanzas.

Review all these *Client receiver methods. They should probably either
all be receivers, or none.

Get rid of Client.Uid.

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

Rename stanzaHandler to something like idCallback. Same for the
HandleStanza function.

Replace inputControl with something like an enum.

Rename In and Out channels to Recv and Send.

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.