New Voronoi implementation based on gorhill/Javascript-Voronoi.

Twice as fast as the old implementation, easier to understand, and handles coincident or collinear points.

Still left to-do:

- [ ] restore voronoi.links.
- [ ] restore voronoi.triangles.
- [ ] restore d3.geom.delaunay.
- [ ] handle coincident points (subsequent points currently have null cells).