Use external ssz library

[iamquang95]

Description

We are self-implementing the ssz logic, which may increase the cost of maintenance.
Sigp team has some good ssz implementation, which we can try and replace our implementation with that

https://github.com/sigp/tree_hash
https://github.com/sigp/ethereum_ssz

[varex83]

What part of ssz you want to replace? If we talk about serialization itself in cluster, we cannot use the libraries, since we need to support multiple versions.
About additional maintenance cost - I think that ssz code won't change a lot in the future

[iamquang95]

Yeah, I just created an issue to take a look at the TreeHash that allows me to write the tree_hash_root without doing it manually
For example

#[derive(TreeHash)]
pub struct DepositData {
    /// Validator's BLS public key (48 bytes)
    pub pub_key: PublicKey,
    /// Withdrawal credentials (32 bytes)
    pub withdrawal_credentials: [u8; 32],
    /// Amount in Gwei to be deposited
    pub amount: Gwei,
    /// BLS signature of the deposit message (96 bytes)
    pub signature: Signature,
}

Instead of manually write

    fn tree_hash_root(&self) -> tree_hash::Hash256 {
        let mut hasher = tree_hash::MerkleHasher::with_leaves(4);

        hasher.write(&self.pub_key).expect("pub_key hash");
        hasher
            .write(&self.withdrawal_credentials)
            .expect("withdrawal_credentials hash");
        hasher
            .write(&self.amount.tree_hash_root().0)
            .expect("amount hash");
        hasher.write(&self.signature).expect("signature hash");

        hasher.finish().expect("hasher finish")
    }

[iamquang95]

Done, tree_hash is adopted.