[SPORK-82] Delta Pruning

[KYLChiu]

Currently only has effect where the evaluator is called MANY times, e.g. at depth 7 for [r1b2rk1/ppq2ppp/3bpn2/3pP3/8/2PB2B1/PP1N1PPP/R2QK2R b KQ - 0 11]:

• test_perf_base 397709282 function calls (396717202 primitive calls) in 132.203 seconds
• test_perf_delta_pruning 352045446 function calls (351229716 primitive calls) in 116.375 seconds
• test_perf_combined 76599863 function calls (76421728 primitive calls) in 25.200 seconds - pretty amazing!

Otherwise it keeps the runtime about the same, though evaluator is called less (this means the extra time to calculate the piece type of captured pieces becomes the bottleneck, I think): [r1b2rk1/ppqn1pbp/6p1/3pp3/7B/2PB1N2/PP3PPP/R2QR1K1 w - - 0 16], depth 4:

test_perf_base        51991010 function calls (51883437 primitive calls) in 16.572 seconds

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
   102263    2.904    0.000    7.676    0.000 evaluator.py:208(evaluate)

test_perf_delta_pruning  45108764 function calls (45028818 primitive calls) in 15.887 seconds

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    74636    2.374    0.000    6.210    0.000 evaluator.py:208(evaluate)