Redefine message length during construction

[mjhouse]

Currently, the entire size of the Message needs to be given during construction, but we may be able to avoid having the size declared using the message generics if we rebuild the message when a new word is added. So the WORD generic argument would default to 0, then a command word is added and the word is rebuilt as Message<2>, then a data word is added, and the word is rebuilt as Message<3> etc.

let message= Message::<3>::new()
        .with_command(CommandWord::new()
        .with_address(Address::Value(12))
        .with_subaddress(SubAddress::Value(5))
        .with_word_count(2)
        .build()?)
    .with_data(DataWord::new())
    .with_data(DataWord::new())
    .build()?;

[mjhouse]

Looks like this will only be possible if we can get WORDS (the generic param to Message) + 1, which isn't easily doable without the generic_const_expr flag, which is currently unstable.

Even with this feature, we would still need a way to return a Message from a constructor function with one of two sizes, which seems difficult to do (cleanly anyway- you might be able to figure out something with enums).