Remove serialization from consensus

[JustinDrake]

Consensus now only cares about Merkleisation (i.e. hash_tree_root), not about serialization (i.e. serialize). This simplifies consensus code by a few tens of lines, is conceptually cleaner, and is more future proof.

A corresponding change in the deposit contract.

[hwwhww]

@JustinDrake
In the current deposit contract, the API is def deposit(deposit_data: bytes[184]), where the input is the serialized DepositData and calculate the leaf value by sha3 in the contract.

So regarding the deposit contract side, my understanding is:

1. Keep having serialization input -> build tree on chain
2. Need to implement tree hash on contract (only for deposit_data case)

Is it what you think?

[JustinDrake]

My suggestion would be to change the deposit contract API to def deposit(pubkey: bytes[48], withdrawal_credentials: bytes[32], amount: bytes[8], signature: bytes[96]) and then manually compute hash_tree_root (which uses sha2) inside the deposit contract.

[hwwhww]

@JustinDrake ahh right that would be more reasonable. 😅

Note that, having the fields in API would limit our options to update eth2 fields without forking eth1 or creating a new deposit contract. Although I remember the last time we discuss the upgradability of the deposit contract, the solution would just be just moving to a new contract, right?

[vbuterin]

Doesn't this mean that we also need to calculate the SSZ root of the deposit data in the Vyper contract?

[JustinDrake]

Yes, we need to calculate hash_tree_root in the contract. Assuming def deposit(pubkey: bytes[48], withdrawal_credentials: bytes[32], amount: bytes[8], signature: bytes[96]) it's something like:

pubkey_root = hash(pubkey + zero_hash[0:16])
signature_root = hash(
    hash(signature[0:64]) +
    hash(signature[64:96] + zero_hash)
)
hash_tree_root = hash(
    hash(pubkey_root + withdrawal_credentials) +
    hash(amount + zero_hash[0:24] + signature_root)
)

[JustinDrake]

the solution would just be just moving to a new contract, right?

Right :)

[hwwhww]

Opened ethereum/deposit_contract#29.
I'll take it on the deposit contract side and then come back to clear and merge this PR.

[hwwhww]

link the contract side: ethereum/deposit_contract#31

[hwwhww]

@djrtwo ethereum/deposit_contract#31 is ready. :)