Type2 accounts' codehash should be computed with Poseidon

[Nashtare]

Accounts stored in Type2 state SMT should have their codehash computed via Poseidon hash instead of Keccak (ref impl).
The method currently available for this is fn hash_bytecode_u256(code: Vec<u8>) -> U256 (though its signature is a bit odd, would be better to change to H256).

@0xaatif would you mind handling this on the decoder side while the cryptos continue fixing the few bits in evm_arithmetization that need work?

[Nashtare]

Note that we should not add a new codehash leaf to the SMT when the associated account's code is empty (unlike what is currently done in evm_arithmetization related branch). Will open an associated ticket for fixing there as it involves rust + asm changes.

[0xaatif]

Does this mean changes to Hash2Code?

[Nashtare]

No actual type change, though the keys would not be computed from Keccak in the case of type2 but poseidon. This refers to changing how code hashes are computed in implementations of the World trait, along with some hashing callsites in core.rs.

I can try tackling tomorrow if you'd prefer.