Deferred effects

Effects that don't happen immediately, but rather contain a condition that's checked after each move and can return _wait_, _cancel_, or _trigger_. So an effect that is meant to happen in 3 turns can contain a condition that compares `state.halfMoves` to the half moves at the time the effect was created +6, and trigger when it is, or wait otherwise. It's also possible to make a variation of this where the effect can be cancelled (I can think of some interesting ideas like a timed mine that explodes after x turns unless destroyed).

This is also needed for Janggi's bikjang rule - flying generals except it happens after the other player's turn if they don't move out of the way (side note: it seems like there are many variations of this rule where it forces a draw or a win or it's optional and so on, so the implementation needs to be flexible).