Add SP-GiST support for range types.

The implementation is a quad-tree, largely copied from the quad-tree
implementation for points. The lower and upper bound of ranges are the 2d
coordinates, with some extra code to handle empty ranges.

I left out the support for adjacent operator, -|-, from the original patch.
Not because there was necessarily anything wrong with it, but it was more
complicated than the other operators, and I only have limited time for
reviewing. That will follow as a separate patch.

Alexander Korotkov, reviewed by Jeff Davis and me.