Variants with secrets

[alexobviously]

Secrets meaning pieces that one or both players don't know about, or pieces that have some flag that isn't visible to the opponent (or both players).

Interesting examples where this would be necessary:
Banqi (https://en.wikipedia.org/wiki/Banqi) - supposedly a Xiangqi variant but actually not really related to it at all. Basically the board is covered in pieces, but neither player knows what any of them are until they're turned over.

Stratego (https://en.wikipedia.org/wiki/Stratego) - kind of like Banqi but the players know what their own pieces are.

Beirut Chess (https://en.wikipedia.org/wiki/Beirut_chess) - each player secretly attaches a bomb to one piece that they can detonate at any time.

This also depends on #30 and maybe also the move refactor