Limit array access in Position

Limit array access in Position

This is a non-functional code style change that routes all position array accesses to single methods, and adds an assert to check correctness.

Passed STC
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 37312 W: 7378 L: 7246 D: 22688
Ptnml(0-2): 606, 4280, 8762, 4392, 616
https://tests.stockfishchess.org/tests/view/5e7c0c69e42a5c3b3ca2eb3d

closes https://github.com/official-stockfish/Stockfish/pull/2595

No functional change.