Major board implementation speedup

Thanks to a few tricks from libego, which include
- with pseudo-liberties now group merging takes O(1)
- atari state of one group can only determine 1 empty
  intersection's hash change (every atari-group is
  next to exactly 1 empty intersection: its own liberty)

we could get a major playout speedup, now 37kpps without UCT(pobench).
This is still with no gamma sampler, which is to come.