Compact form EIP-2098 signature recovery

[iqdecay]

From @Braqzen

Motivation

In Ethers-rs (and I assume it's also in Ethers-js but I have not gotten there yet), when a message is signed the return type is a Signature which when converted into a vector consists of 65 bytes that represent the r, s and v values.

The issue is that Sway uses 64 bytes.
There is a function that can be implemented to transform the representation here (to_compact()).

This is not the final representation because it's not a single vector of bytes but we can have that function and perhaps another which takes these two values and outputs a vector.

Possible implementations

This is an implementation I have written in Rust inspired from Ethers-rs. I have not tested this to prove that it works but I think it might. It may be taken as an example from Rust and ported to Ts (and also added into the Rust SDK).

fn compact(signature: &Signature) -> [u8; 64] {
    let shifted_parity = U256::from(signature.v - 27) << 255;

    let r = signature.r;
    let yParityAndS = shifted_parity | signature.s;

    let mut sig = [0u8; 64];
    let mut r_bytes = [0u8; 32];
    let mut s_bytes = [0u8; 32];
    r.to_big_endian(&mut r_bytes);
    yParityAndS.to_big_endian(&mut s_bytes);
    sig[..32].copy_from_slice(&r_bytes);
    sig[32..64].copy_from_slice(&s_bytes);
    return sig;
}