Adjust LMR based on critical chain length

[cj5716]

This patch is inspired by the principle of Heavy Light Decomposition. Instead, we define a heavy edge as the one where we played our most promising option. We then adjust the LMR reduction based on the length of our current current chain of "best moves", which we use as a prediction of the node's value (and thus reduce less on its branches accordingly).

STC:
Elo | 2.03 +- 1.56 (95%)
SPRT | 8.0+0.08s Threads=1 Hash=16MB
LLR | 3.08 (-2.25, 2.89) [0.00, 3.00]
Games | N: 51114 W: 12980 L: 12681 D: 25453
Penta | [183, 5955, 13010, 6198, 211]
https://recklesschess.space/test/15642/

LTC:
Elo | 1.82 +- 1.44 (95%)
SPRT | 40.0+0.40s Threads=1 Hash=64MB
LLR | 2.93 (-2.25, 2.89) [0.00, 3.00]
Games | N: 51310 W: 12743 L: 12474 D: 26093
Penta | [21, 5695, 13954, 5964, 21]
https://recklesschess.space/test/15643/

Bench: 2894755