Add MaxCborLen trait and derive macro for compile-time max CBOR size

[menasheofd]

Add MaxCborLen trait and #[derive(MaxCborLen)]

Adds compile-time maximum CBOR encoding size calculation, useful for
fixed-size stack buffers in no_std without alloc:

#[derive(Encode, MaxCborLen)]
#[cbor(array)]
struct Msg {
    #[n(0)] sensor_id: u8,
    #[n(1)] reading: u32,
}

let mut buf = [0u8; Msg::MAX_CBOR_LEN];
minicbor::encode(&msg, buf.as_mut_slice()).unwrap();

[kotfind]

+1 for this feature.

I also think an integration with typenum crate would be cool. Currently rust's support for const generics in quite limited, but typenum + generic-array often solve this issue.