3x faster, 25% more memory

$ /usr/bin/time ./little_bishops.prev.py 8 6
COUNT: 5599888
161.56user 0.00system 2:41.43elapsed 100%CPU (0avgtext+0avgdata 5148maxresident)k
0inputs+0outputs (0major+1621minor)pagefaults 0swaps

$ /usr/bin/time ./little_bishops.py 8 6
COUNT: 5599888
54.91user 0.00system 0:54.87elapsed 100%CPU (0avgtext+0avgdata 6472maxresident)k
0inputs+0outputs (0major+1946minor)pagefaults 0swaps