Improve public key storage efficiency

[tarrencev]

Currently, the users credential private key coordinates are naively stored as 86bit limbs, which is what the signature verification expects as input.

We store 3 limbs for both x, y coordinates, we're using six felts for storage. Since 6 * 86 = 516 and a felt can store 250bits, we should instead represent the key as 3 felts.

[piniom]

Hi @tarrencev, Is this issue still relevant? Currently we store webauthn pub_key as four felts here:

struct WebauthnPubKey {
    x: u256,
    y: u256,
}