EIP-1344: Add chain id opcode

Hi @fubuloubu @fulldecent
I created a new EIP for my VALID_CHAINID opcode proposal with an extensive rationale of why I think it is better than the CHAINID opcode proposed here, at least for the goal of protecting users of off-chain messages. And why we should include it instead of this one.

Discussion here : EIP-1959 Valid ChainID opcode