Corrplexity for futility pruning

Add corrhist-based term to futility margin

Inspired by a recent patch of Shawn Xu, this tweak increases the margin
over beta needed to futility prune based on the correction history, with
an offset.

src/search.cpp

@@ -698,6 +698,7 @@ Value Search::Worker::search(
     if (!ss->ttPv && depth < 16
         && eval - futility_margin(depth, cutNode && !ss->ttHit, improving, opponentWorsening)
                - (ss - 1)->statScore / 159
+               + (ss->staticEval == eval) * (40 - std::abs(correctionValue) / 131072)
              >= beta
         && eval >= beta && (!ttData.move || ttCapture) && !is_loss(beta) && !is_win(eval))
         return beta + (eval - beta) / 3;