21 bit tt keys with SWAR
This patch restructures TT clusters and extends the TT keys inside the cluster from 16 to 21 bits (thus using 15 of the unused 16 bits in the old layout). This should make hash collisions 32 times less likely.
This could improve the frequency with which issues such as https://github.com/official-stockfish/Stockfish/issues/7221 and https://github.com/official-stockfish/Stockfish/issues/5174 occur
passed non-reg STC: https://tests.stockfishchess.org/tests/view/6ad492fb733e627f7058c711
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 51552 W: 13180 L: 12986 D: 25386
Ptnml(0-2): 59, 5703, 14059, 5895, 60
passed non-reg LTC: https://tests.stockfishchess.org/tests/view/6ad70b25733e627f7058cda3
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 71394 W: 18358 L: 18195 D: 34841
Ptnml(0-2): 16, 7341, 20819, 7506, 15
also passed non-reg SMP STC with high hash pressure: https://tests.stockfishchess.org/tests/view/6ad71654733e627f7058cdb5
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 107224 W: 27527 L: 27390 D: 52307
Ptnml(0-2): 184, 11916, 29288, 12027, 197
closes https://github.com/official-stockfish/Stockfish/pull/7236
Bench: 2389370