Use set_turn() to change sides in submit_move()
Use set_turn() to change sides in submit_move()

The method Board::set_turn() must be called before Move::side_effects()
so that Pawn_Double_Move::side_effects() can set the en_passant_target
square without it being overwritten by set_turn().

MarkZH/Genetic_Chess,MarkZH/Genetic_Chess,MarkZH/Genetic_Chess,MarkZH/Genetic_Chess,MarkZH/Genetic_Chess