Implement special moves — castling, en passant, promotion

[MoMath3000]

Question mark - I know the conditions for each of these, but I need to figure out where and how to implement them.

[MoMath3000]

• Castling (kingside/queenside, requires king & rook moved == false, empty squares between, no check along the path)
• En passant
• Pawn promotion