Return number of bytes written to buffer from encode()

[MathiasKoch]

This PR add a return value to encode(), in order to allow using it with types that implement BufMut without keeping track of length, such as raw byte slices as

let mut tx_buf: [u8; 2048] = [0; 2048];
let size = encode(&pkt, &mut &mut tx_buf[..])?;
network.write(socket, &tx_buf[..size]);

[00imvj00]

thank you @MathiasKoch