Make EIP712 functionality compatible with delegate calls

[k06a]

Consider borrowing the idea from this PR: OpenZeppelin/openzeppelin-contracts#2852

You have to cache the address of the smart contract in immutable to prevent issues when implementation is used via delegate calls.

[transmissions11]

would prefer ppl fork, don't want to increase complexity for a use case we don't encourage

[k06a]

@transmissions11 someone could use your library in this manner and get suffered from this issue. I assume the fix is relatively cheap, just cache address of this in immutable.

[transmissions11]

There are plenty of other foot guns, such as the reentrancy guard not being compatible with delegatecall