Add builder APIs

[vincentdephily]

Currently, Packet construction can only be done by filling every field, which is neat but a bit wordy:

let pkt = Connect { protocol: Protocol::MQTT311,
                        keep_alive: 60,
                        client_id: "foo".into(),
                        clean_session: true,
                        last_will: None,
                        username: None,
                        password: None };

It'd be nice to have [#derive(Default)] or impl fn new() -> Self and a full set of fn set_clientid(self, String) -> Self.

Just need to make sure that packets have reasonable defaults (provide a new() with arguments otherwise) and that using these API don't cause extra allocations.

It might also be a good time to get a closer look at MQTT5 to see how mqttrs will transistion to supporting both.