Small tidy up in move_to_san

No functional change.
Merge branch 'master' into imbalance
Revert "Remove imbalance completely (9174797)"

This reverts commit 3140a3437c78a6344989223f2049ab619f3913a8.
Simplified imbalance (7163197)
- Bishop pair, which is obviously the most important feature. Use about 1/2 endgame pawn for this.
- Minor difference multiplied by total number of pawns. This formula is from DiscoCheck, and
originates in a simplification of Thomas Kolarik's formula, used in his engine Nirvana.