Tsumego trainer

Every problem here is checked against the real rules kernel — your answer is only accepted once it's actually replayed and legal, and reaching the stated goal (capture, escape, life, or a kill) is verified the same way, not just pattern-matched against a pixel-coordinate. Reviews are scheduled with a spaced-repetition algorithm (see the README for provenance): grade yourself honestly after each problem and the trainer will bring it back right around when you're about to forget it.