Use a std::bitset for KPKBitbase

Use a std::bitset for KPKBitbase

This is a non-functional simplification. Looks like std::bitset works good
for the KPKBitbase. Thanks for Jorg Oster for helping get the speed up
(the [] accessor is faster than test()).

Speed testing: 10k calls to probe:
  master 9.8 sec
  patch 9.8 sec.

STC
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 100154 W: 19025 L: 18992 D: 62137
Ptnml(0-2): 1397, 11376, 24572, 11254, 1473
http://tests.stockfishchess.org/tests/view/5e21e601346e35ac603b7d2b

Closes https://github.com/official-stockfish/Stockfish/pull/2502

No functional change