Implement RFC 4361

[leshow]

https://www.rfc-editor.org/rfc/rfc4361

allows a v6-style DUID to be encoded in the client id option (opt 61). We should probably change the ClientId option so that it is easier to pull out the DUID specified here: https://www.rfc-editor.org/rfc/rfc4361#section-6.1

maybe:

enum ClientId {
    Duid(Duid),
    Id(Id)
}
struct Duid {
   iaid: [u8; 4],
   duid: Vec<u8>
}
struct Id {
   id: Vec<u8>
}

With some convenience methods on ClientId to pull out either id or duid if a method is called.