Refactor/(generic bufsize)

[MathiasKoch]

This PR allows creating ATAT instances with generic receive buffer size.

Building the Client/IngressManager pair through ClientBuilder will keep the default buffer length of U256, but it allows specifying lengths if crated "manually" as

let (res_p, res_c) = unsafe { RES_QUEUE.split() };
let (urc_p, urc_c) = unsafe { URC_QUEUE.split() };
let (com_p, com_c) = unsafe { COM_QUEUE.split() };

let at_config = Config::new(Mode::Timeout);
let mut ingress: IngressManager<consts::U512, NoopUrcMatcher> = IngressManager::with_custom_urc_matcher(
    res_p,
    urc_p,
    com_c,
    at_config,
    None,
);
let client: Client<Tx<UART4>, Timer, consts::U512> = Client::new(tx, res_c, urc_c, com_p, cell_at_timer, at_config);

Note This is still a bit of a work-in-progress, as i am not super happy with the API breaking it brings, though i don't think there are better ways?

It will address the part of #43 regarding IngressManager - buf capacity

Any better ideas will be welcomed with great joy!

[MathiasKoch]

Updated the PR to actually fix #43 , while also introducing a driver! macro, to replace the previous ClientBuilder.. This also solves part of the issue of instantiating multiple clients.. We still need a way of building unique identifiers for the static queues in the macro.. Maybe the only way is to introduce driver! as a proc_macro.