Use a big Span<Move> to save moves instead of move pool.

[eduherminio]

A realPly variable is introduced, since we need to offset the moves every time we search deeper to avoid overriding previous, existing searches A different override of Span.Sort is used, the one that uses an array of keys instead of a comparison lambda.

#355 but squashed, to preserve the changes in one single commit.

Score of Lynx 1282 - span vs Lynx 1279 - main: 1887 - 2008 - 1168  [0.488] 5063
...      Lynx 1282 - span playing White: 1141 - 798 - 592  [0.568] 2531
...      Lynx 1282 - span playing Black: 746 - 1210 - 576  [0.408] 2532
...      White vs Black: 2351 - 1544 - 1168  [0.580] 5063
Elo difference: -8.3 +/- 8.4, LOS: 2.6 %, DrawRatio: 23.1 %
SPRT: llr -2.95 (-100.0%), lbound -2.94, ubound 2.94 - H0 was accepted