Add EIP-712

[fubuloubu]

It's important for the safety of user funds to use a domain separation technique in order to protect the signing context of a person's account, especially if the OVM technique is to see wider use. EIP-712 has a lot of adoption (metamask and other wallets have integrated, dedicated opcode for chain ID in Istanbul, currently being finalized, etc.) so it would be a great suggestion to manage domain separation of the meta-txns/L2 signed messages that OVM produces. It is fairly easy to integrate, requiring only a marginal growth in the complexity of signature verification in on-chain smart contracts. However, it would give a stable API endpoint that can be targeted in Web3 providers to ensure the safety of user keys, which is a much larger benefit.