Replace RTree with RBush

Replaces the old RTree with [RBush](https://github.com/mourner/rbush), my new high-performance R-Tree implementation.

Everything seems to work smoothly and probably faster than before, but I don't know how to measure the exact performance impact — if you do, I'd be happy to hear the results.

One more thing worth exploring is bulk insertion for the labels code which is not used here because at the moment every insertion is dependent on results of previous ones (`tryInsert` function). If you can reorganize the code to batch unrelated insertions, we can use bulk insertion and increase performance.

cc @jfirebaugh @tmcw