Improved Killer and Null Move Heuristics

• The Killer Moves table now updates every time a cutoff occurs, instead of keeping the first 2 values.
  This generally means that deeper moves will replace the old ones.
  Also, instead of clearing the table after each search, it now gets shifted by 2 plies, keeping the moves found in previous searches. This change increased the number of alpha-beta cutoffs caused by killer moves.
• The Null Move heuristic received a small tweak that greatly improved it: previously, it was limitied to depth>3.
  Removing this limit vastly increased the number of alpha-beta cutoffs it achieved. This in turn yielded an increase to NPS, without apparent negative effects.
• The engine now prints stats like NPS, alpha-beta cutoffs, etc., which helped discover the improvements in this version.

First version

Includes:

• Magic bitboards
• Principal Variation Search
• Quiescence Search
• ABDADA multithreading
• Transposition Table
• Null Move Heuristic
• Killer Move Heuristic
• Late Move Reductions