core/types: change SetCodeTx.ChainID to uint256

[fjl]

We still need to decide how to handle non-specfic chainId in the JSON encoding of authorizations. With chainId being a uint64, the previous implementation just used value zero. However, it might actually be more correct to use the value null for this case.

[fjl]

Hmm. So now it's a bit inconsistent with SetCodeAuthorization having ChainID of type uint256.Int and the transaction type having a ChainID of type *uint256.Int. I think the non-pointer version is more correct since we treat the value zero as 'valid for all chains' for authorizations.

[lightclient]

Why do we use *uint256.Int for other instances of chain id? I always assumed it was because the pointer is just less data for shuttle around. AFAIK the chain id is required for every tx anyways.

We still need to decide how to handle non-specfic chainId in the JSON encoding of authorizations. With chainId being a uint64, the previous implementation just used value zero. However, it might actually be more correct to use the value null for this case.

In terms of this, it seems like encoding as 0 is the right way to represent valid on all chains since that is the specified value in the EIP.