HandshakeState: Add function to find out size of next write

[fogti]

It would be useful to find out which size (w/o payload) the next HandshakeState::write_message will produce, so that the payload may be used to pad up the message to make it harder for an attacker to find out that a re-handshake happened (which means that I can't simply save the length of the padding outside of the more-or-less-encrypted payload, as it would be noticable w/o decrypting it).

e.g. I'm currently able to pad transport messages (https://github.com/YZITE/encsess2/blob/297b279a9274750bb83d9c5ab7a3ea3cfee29184/lib/src/lib.rs#L106-L135), but because the size of handshake messages depends on the handshake pattern, vary between different handshake messages (e.g. are at least different for initiator and responder) and I really don't want to hardcode them, an API for that would be necessary.

idea:

const PAD_TRG_SIZE: usize = 64;
let mut noise: HandshakeState = ...;
// ...
if noise.is_my_turn() {
    let next_msg_len = noise.simulate_write_message(&[]).unwrap();
    let padding_len = PAD_TRG_SIZE - (next_msg_len % PAD_TRG_SIZE);
    let mut padding = Vec::new();
    padding.resize(padding_len);
    rand::RngCore::fill_bytes(&mut rand::thread_rng(), &mut padding[..]);
    let mut hsmsg = [0u8; 0xffff];
    let real_len = noise.write_message(&padding[..], &mut hsmsg[..]).unwrap();
    assert_eq!(real_len, next_msg_len + padding_len);
    // send hsmsg via network...
}