Use evaluation trend to adjust futility margin

Adjust futility margin in the child node based on whether the
static evaluation is improving.

STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 15271 W: 3157 L: 2958 D: 9156
http://tests.stockfishchess.org/tests/view/5a9f2f8c0ebc590297cb6216

LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 6617 W: 1053 L: 908 D: 4656
http://tests.stockfishchess.org/tests/view/5a9f98390ebc590297cb6241

Ideas for future work:

- Tune the new margins.
- Try to get this idea to work for futility pruning in parent
  nodes as well.

Bench: 5779242