smite: add revoke_and_ack message codec implementation

[kartikangiras]

Add support for Revoke Ack message codec.
relates to #5

[kushagra0902]

The PR in general looks good and has all the fields from the Bolt-02 type 133. There is a small naming/intent problem in one of the tests mentioned.

[kushagra0902]

The test does not generate different public keys; it uses the same constant key in every iteration, so the test name is misleading, as the code let pk = PublicKey::from_slice(&[0x02; 33]).expect("valid public key"); Generates same keys in every iteration
It would be better to use this:

let sk1 = SecretKey::from_slice(&[0x01; 32]).unwrap();
    let sk2 = SecretKey::from_slice(&[0x03; 32]).unwrap();

    for sk in &[sk1, sk2] {
        let pk = PublicKey::from_secret_key(&secp, sk);