Add quadtree.find.
Simplify quadtree.find and add test.
Minor optimisation for quadtree.find.

Reduces branching a bit more.
Save a few bytes.
Save a few more bytes.

The removal of `k = i & 3` also seems to produce a slight speedup in
Firefox 29.0.1 according to a microbenchmark.
Merge remote-tracking branch 'origin/quadtree-find' into 3.5