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:

- [x] restore voronoi.links.
- [x] restore voronoi.triangles.
- [x] restore d3.geom.delaunay.
- [ ] handle coincident points (subsequent points currently have null cells).
- [x] get tests working again.
- [x] eliminate duplicate Liang–Barsky line clipping with geo/clip-extent.js.