7-pieces Syzygy tablebase support

7-pieces Syzygy tablebase support

This is the first patch teaching Stockfish how to use the 7-pieces
Syzygy tablebase currently calculated by Bujun Guo (@noobpwnftw) and
Ronald de Man (@syzygy1). The 7-pieces database are so big that they
required a change in the internal format of the files (technically,
some DTZ values are 16 bits long, so this had to be stored as wide
integers in the Huffman tree).

Here are the estimated file size for the 7-pieces Syzygy files,
compared to the 151G of the 6-pieces Syzygy:

```
7.1T    ./7men_testing/4v3_pawnful (ongoing, 120 of 325 sets remaining)
2.4T    ./7men_testing/4v3_pawnless
2.3T    ./7men_testing/5v2_pawnful
660G    ./7men_testing/5v2_pawnless
117G    ./7men_testing/6v1_pawnful
87G     ./7men_testing/6v1_pawnless
```
Some pointers to download or recalculate the tables:

Location of original files, by Bujun Guo:
ftp://ftp.chessdb.cn/pub/syzygy/

Mirrors:
http://tablebase.sesse.net/ (partial)
http://tablebase.lichess.ovh/tables/standard/7/

Generator code:
https://github.com/syzygy1/tb/

Closes https://github.com/official-stockfish/Stockfish/pull/1707

Bench: 5591925 (No functional change if SyzygyTB is not used)

----------------------

Comment by Leonardo Ljubičić (@DragonMist)

This is an amazing achievement, generating and being able to use 7 men syzygy
on the fly. Thank you for your efforts @noobpwnftw !! Looking forward how this
will work in real life, and expecting some trade off between gaining perfect
play and slow disc Access, but once the disc speed and space is not a problem,
I expect 7 men to yield something like 30 elo at least.

-----------------------

Comment by Michael Byrne (@MichaelB7)

This definitely has a bright future. I turned off the 50 move rule (ala ICCF
new rules) for the following position:  `[d]8/8/1b6/8/4N2r/1k6/7B/R1K5 w - - 0 1`
This position is a 451 ply win for white (sans the 50 move rule, this position
was identified by the generator as the longest cursed win for white in KRBN v KRB).

Now Stockfish finds it instantly (as it should), nice work 👊👍 .
```
dep score	    nodes	    time
  7	+132.79 	4339    	0:00.00	Rb1+ Kc4 Nd6+ Kc5 Bg1+ Kxd6 Rxb6+ Kc7 Be3 Rh2 Bd4
  6	+132.79 	1652    	0:00.00	Rb1+ Kc4 Nd2+ Kd5 Rxb6 Rxh2 Nf3 Rf2
  5	+132.79 	589      	0:00.00	Rb1+ Kc4 Rxb6 Rxh2 Nf6 Rh1+ Kb2
  4	+132.79 	308      	0:00.00	Rb1+ Kc4 Nd6+ Kc3 Rxb6 Rxh2
  3	+132.79 	88        	0:00.00	Rb1+ Ka4 Nc3+ Ka5 Ra1+ Kb4 Ra4+ Kxc3 Rxh4
  2	+132.79 	54        	0:00.00	Rb1+ Ka4 Nc3+ Ka5 Ra1+ Kb4
  1	+132.7
```