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.
351fc0cce2