Provide opt-in variable length integer encoding

[dtolnay]

Maybe something like:

#[derive(Serialize, Deserialize)]
struct Example {
    #[serde(with = "bincode::varint")]
    a: usize,
    #[serde(with = "bincode::varint")]
    b: i32,
}

[whimboo]

To implement eg. the ADB protocol an encoded length of 4 bytes is used. @dtolnay, would that be possible to define via this issue request, or is it something else?

[Tolsi]

PTAL