Rules that modify player perspectives (e.g. fog of war)

[alexobviously]

Although it's technically possible to do this sort of thing (and maybe implement it in Square Bishop), it would be useful to implement this sort of thing in Bishop itself, so that variants could define this sort of behaviour.

I'm not sure exactly how much work this is. I think it's mostly going to be just definitions in variants about how perspective processing should be applied, and output functions. What we probably want is a function that provides a user-facing version of the game state that goes through a filter, which applies the processing. This will probably require a rework of Square Bishop but that's fine, and it will likely be an improvement anyway.

https://www.chess.com/terms/fog-of-war-chess - an interesting first use case

Depends on #40

[alexobviously]

This is done in 1.3.3 with the new StateTransformer, though there currently isn't an exact fog of war transformer yet, because that depends on some move gen refactors.