Support rules relating to perpetual checks

[alexobviously]

This can be handled by a new event: ActionEvent.onCheck. Probably the simple way is to allow analysing the state history in actions, but this would require linking states or something like that, maybe there's some other way of tracking checks. I'm averse to putting more specific case fields in BishopState though so maybe linking is the best way (as in, BishopState has a field linking to its previous state).

This handles part of (but not all of) #16

[alexobviously]

We have checks in state now, I wonder if we can safely give states parents and traverse..