Expose the type as a static method with the derive macros

[vincenzopalazzo]

We currently use the following code to derive and match the type.

#[derive(DecodeWire, EncodeWire, Debug)]
pub struct Init {
    #[warn(dead_code)]
    #[msg_type=16]
    ty: u16,
    globalfeatures: BitFlag,
    features: BitFlag,
    init_tlvs: Stream,
}

Unfortunately, we cannot access the message type using a static method like Init::type(). This feature would be especially useful when working with a generic parser on the command line.