Add lib/rhill-voronoi.
Checkpoint new Voronoi implementation.

Still significant work left to minimize the size of the implementation, and
restore support for Delaunay meshes, but this provides a starting point.
Restructure Voronoi implementation using SMASH.
Minor polish.
Use a simple [x, y] array for vertices.
Shorten member names.
More shorter names.
More shorter names.
Shorter words.
More shortening.
Use clipExtent instead of bbox.
Shorter words.
Shorter words.
Alias Math.abs.
Remove obsolete license.
Use null rather than undefined.
Shorter.
Fix leak when removing nodes from rb-tree.
Merge files.
Shorter.