Add Voronoi benchmark.

test/geom/voronoi-benchmark

@@ -0,0 +1,24 @@
+#!/usr/bin/env node
+
+var d3 = require("../../");
+
+var width = 960,
+    height = 500;
+
+var points = d3.range(10000).map(function() {
+  return [Math.random() * width, Math.random() * height];
+});
+
+var voronoi = d3.geom.voronoi()
+    .clipExtent([[0, 0], [width, height]]);
+
+var observations = d3.range(20).map(function() {
+  process.stdout.write(".");
+  var start = process.hrtime();
+  voronoi(points);
+  var elapsed = process.hrtime(start);
+  return elapsed[0] * 1e3 + elapsed[1] / 1e6;
+});
+
+console.log("");
+console.log(Math.round(d3.mean(observations)) + "ms for 10,000 points.");