Further documentation and coding style on TB code

Further documentation and coding style on TB code

This patch adds some documentation and code cleanup to tablebase code.

It took me some time to understand the relation among the differrent
structs, although I have rewrote them fully in the past. So I wrote
some detailed documentation to avoid the same efforts for future readers.

Also noteworthy is the use a standard hash table implementation with a
more efficient 1D array instead of a 2D array. This reduces the average
lookup steps of 90% (from 343 to 38 in a bench 128 1 16 run) and reduces
also the table from 5K to 4K
entries.

I have tested on 5-men and no functional and no slowdown reported. It
should be verified on 6-men that the new hash does not overflow. It is
enough to run ./stockfish with 6-men available: if it does not assert at
startup it means everything is ok with 6-men too.

EDIT: verified for 6-men tablebase by Jörg Oster. Thanks!

No functional change.