Check if the king of the current player is in check
Switch players
Check if the current player owns the piece on a given position
Move a piece
Redefine Rook#move
Fix King#valid_move?
Redefine King#move
Redefine Pawn#move
Define ChessBoard#make_a_move
Define functions for the different game outcomes
Handle pawn promotion
Get the color of the current player