ChessEngine: Use reduced Board objects in MCTSTree

Monte-Carlo methods are known to intensively create nodes and in our
case each leaf of the tree stores a board. However, for this use case,
we don't need a full board object that also contains game information.

This patch adds a `clone_cleared()` method that return a clone without
game information and uses it when constructing the tree.
It allows the ChessEngine much more possibility before getting out of
memory.

Userland/Libraries/LibChess/Chess.h

@@ -117,6 +117,7 @@ struct Move {
 class Board {
 public:
     Board();
+    Board clone_without_history() const;
 
     Piece get_piece(Square const&) const;
     Piece set_piece(Square const&, Piece const&);