Xiangqi flying generals rule about bishop

[alexobviously]

I'm starting to think this should be part of a more generalised state checking system that allows for more complex conditions on moves, but I'll have to check how this would affect performance - certainly such a system should exist, but should the xiangqi case be a special case we optimise for?

Perhaps a good way to do such a thing is for variants to define a list of checks, which have conditions built in (i.e. the move generator only calls the check if conditions are met, like "after a move of piece type K, we call this check to see if the other king is on the same file with no pieces in between". This would also be a nice way of starting to delegating some of the logic calls into BuiltVariant, which is ultimately my goal.

[alexobviously]

Complete as of #356eb4 - actions update