Use prototypes for d3.rgb() and d3.hsl().

This turns out to be slightly faster than using structs (`{}`) and is bit
cleaner.  You can now also do:

    d3.rgb(r, g, b).hsl()

and:

    d3.hsl(h, s, l).rgb()

to convert between RGB and HSL.

d3.js

@@ -707,14 +707,20 @@ d3.rgb = function(r, g, b) {
 };
 
 function d3_rgb(r, g, b) {
-  return {
-    r: r, g: g, b: b,
-    toString: d3_rgb_format,
-    brighter: d3_rgb_brighter,
-    darker: d3_rgb_darker
-  };
+  return new d3_Rgb(r, g, b);
+}
+
+function d3_Rgb(r, g, b) {
+  this.r = r;
+  this.g = g;
+  this.b = b;
 }
 
+d3_Rgb.prototype.toString = d3_rgb_format;
+d3_Rgb.prototype.brighter = d3_rgb_brighter;
+d3_Rgb.prototype.darker = d3_rgb_darker;
+d3_Rgb.prototype.hsl = d3_rgb_hsl;
+
 function d3_rgb_brighter(k) {
   k = Math.pow(0.7, arguments.length ? k : 1);
   var r = this.r,
@@ -991,9 +997,18 @@ d3.hsl = function(h, s, l) {
 };
 
 function d3_hsl(h, s, l) {
-  return {h: h, s: s, l: l, toString: d3_hsl_format};
+  return new d3_Hsl(h, s, l);
 }
 
+function d3_Hsl(h, s, l) {
+  this.h = h;
+  this.s = s;
+  this.l = l;
+}
+
+d3_Hsl.prototype.toString = d3_hsl_format;
+d3_Hsl.prototype.hsl = d3_hsl_rgb;
+
 /** @this d3_hsl */
 function d3_hsl_format() {
   return "hsl(" + this.h + "," + this.s * 100 + "%," + this.l * 100 + "%)";

src/core/hsl.js

@@ -9,9 +9,18 @@ d3.hsl = function(h, s, l) {
 };
 
 function d3_hsl(h, s, l) {
-  return {h: h, s: s, l: l, toString: d3_hsl_format};
+  return new d3_Hsl(h, s, l);
 }
 
+function d3_Hsl(h, s, l) {
+  this.h = h;
+  this.s = s;
+  this.l = l;
+}
+
+d3_Hsl.prototype.toString = d3_hsl_format;
+d3_Hsl.prototype.hsl = d3_hsl_rgb;
+
 /** @this d3_hsl */
 function d3_hsl_format() {
   return "hsl(" + this.h + "," + this.s * 100 + "%," + this.l * 100 + "%)";

src/core/rgb.js

@@ -9,14 +9,20 @@ d3.rgb = function(r, g, b) {
 };
 
 function d3_rgb(r, g, b) {
-  return {
-    r: r, g: g, b: b,
-    toString: d3_rgb_format,
-    brighter: d3_rgb_brighter,
-    darker: d3_rgb_darker
-  };
+  return new d3_Rgb(r, g, b);
 }
 
+function d3_Rgb(r, g, b) {
+  this.r = r;
+  this.g = g;
+  this.b = b;
+}
+
+d3_Rgb.prototype.toString = d3_rgb_format;
+d3_Rgb.prototype.brighter = d3_rgb_brighter;
+d3_Rgb.prototype.darker = d3_rgb_darker;
+d3_Rgb.prototype.hsl = d3_rgb_hsl;
+
 function d3_rgb_brighter(k) {
   k = Math.pow(0.7, arguments.length ? k : 1);
   var r = this.r,