quadtree: add Collect method for debugging
quadtree: add Collect method for debugging

This shouldn't ever be used for anything but debugging. The intent is to
be able to walk the quadtree and gather all the items stored inside it
and compare against what was expected to be in it.

Signed-off-by: Steven Noonan <4068f0880b399410602d694b3cc711c8a8f4727e@uplinklabs.net>

tycho/crisscross,tycho/crisscross,tycho/crisscross