Support rules relating to perpetual checks

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