Refactor d3.geom.{voronoi,quadtree}.

Experimenting with the ideas discussed in #1145.  Notes:

* delaunay is deprecated by voronoi.triangles.
* hull should be refactored similarly.
* Should polygon be refactored to use accessors too?
* If so, polygon should also be consistent with voronoi in the way it supports additional functionality (area, centroid; triangles, links).
* Related to the previous point, perhaps voronoi should reuse the same voronoi diagram for all three methods (voronoi, triangles, links).

(Less important, is it a good idea to import ../svg/line?  Perhaps these default accessors should be moved to ../core/point or similar.)