Chess engine: Stockfish 20082412 and NNUE

Stockfish - UCI chess engine, compiled by Sami Kiminki
Rating JCER=3307

Timestamp: 1598263408

Allow TT entries with key16==0 to be fetched

Fix the issue where a TT entry with key16==0 would always be reported as a miss. Instead, we'll use depth8 to detect whether the TT entry is occupied. In order to do that, we'll change DEPTH_OFFSET to -7
(depth8==0) to distinguish between an unoccupied entry and the otherwise lowest possible depth, i.e., DEPTH_NONE (depth8==1).

To prevent a performance regression, we'll reorder the TT entry fields by the access order of TranspositionTable::probe(). Memory in general
works fastest when accessed in sequential order. We'll also match the store order in TTEntry::save() with the entry field order, and re-order the 'if-or' expressions in TTEntry::save() from the cheapest to the most expensive.

Finally, as we now have a proper TT entry occupancy test, we'll fix a minor corner case with hashfull reporting. To reproduce:
- Use a big hash
- Either:
a. Start 31 very quick searches (this wraparounds generation to 0); or
b. Force generation of the first search to 0.
- go depth infinite

Before the fix, hashfull would incorrectly report nearly full hash immediately after the search start, since
TranspositionTable::hashfull() used to consider only the entry
generation and not whether the entry was actually occupied.

STC:
LLR: 2.95 (-2.94,2.94) {-0.25,1.25}
Total: 36848 W: 4091 L: 3898 D: 28859 Elo +1.82
Ptnml(0-2): 158, 2996, 11972, 3091, 207
https://tests.stockfishchess.org/tests/view/5f3f98d5dc02a01a0c2881f7

LTC:
LLR: 2.95 (-2.94,2.94) {0.25,1.25}
Total: 32280 W: 1828 L: 1653 D: 28799 Elo +1.88
Ptnml(0-2): 34, 1428, 13051, 1583, 44
https://tests.stockfishchess.org/tests/view/5f3fe77a87a5c3c63d8f5332

Bench: 3760677

Stockfish 20082412 and NNUE download