Rectangle efficiency

[eadwright]

I'm creating a large RTree with about 10M points. I wanted to use a flyweight pattern for the geometry, but alas I can't because 1) Util.mbr() creates a new Rectangle instance instead of using my geometry directly, and 2) Rectangle is a class not an interface with an implementation.

Why does this matter? The Rectangle instances take 600MB of my heap whereas the underlying (point) data takes less than 200MB. If Rectangle was an interface with a default implementation, and if Util could use it directly when the items collection only has 1 item in it, that could have a big performance gain in terms of memory. Done right, it wouldn't hurt speed in any significant way.

[eadwright]

Fixed here: #56