Tag printing in rbtree.check() with debug to enable purity inference

The calls to `write` and `writeln` in member `check` prevents `rbtree`'s members from begin `pure`. Adding `debug` is one way of fixing this. I bet you guys think differently but I anyway hope that you all agree on that purity is better than pretting printing in a check member function. One alternative solution would be to make `printTree` return a string which is then fed to `throw new Exception`. Destroy! :)
You don't need to debug call printTree from itself :)