Handle IpseckeyGateway::None branch in hash impl

[SebastiaanYN]

Replace the hand-written hash implementation with #[derive(Hash)] to remove the todo!() from the None branch.

[partim]

I think IpseckeyGateway needs to hash safely on its own without relying on information from Ipseckey. This means that the current implementation is also broken. I suspect we can just derive Hash?

[SebastiaanYN]

Good point, that's a lot easier. I've updated the PR