Enhance search if a node is completely inside the query box.

[muendlein]

If I understand the code correctly, the leaves belonging to a node are a continous array segment. This should allow for some improvements if the query box covers most of the data.

[mourner]

Good call! This would be an equivalent of this change in RBush mourner/rbush#11. It might get a little bit more verbose in Flatbush case, but worth exploring.