minor perf improvements and testing

- added functions to test single level gpu perft speeds
-- max speed: 17BNps @ start pos, 32.5 BNps @ pos 2 (best case)
-- max speed without divergence @ pos2: 43.7 BNps (best case)
-- looks bottlenecked by occupancy (limited by num of regs). Performance
slightly improves by setting MAX_REGS = 64 for pos2.
- tried out multiple (recursive) levels of move generation near leaf
node.
-- improves performance at start position (~7.2 BNps)
-- reduces performance at pos2 (too much divergence?)
- add separate kernel to generates moves at leaf nodes
- current performance ~11 BNps at pos2 (best case)