Memoize color object serialization

Still not efficient enough as color object are frequently created
and each will need to be serialized once regardless of how many
times it is used.

Author: limzykenneth

src/color/p5.Color.js

@@ -33,6 +33,8 @@ import HSBSpace from './color_spaces/hsb.js';
 const map = (n, start1, stop1, start2, stop2) =>
   ((n - start1) / (stop1 - start1) * (stop2 - start2) + start2);
 
+const serializationMap = new WeakMap();
+
 class Color {
   // Reference to underlying color object depending on implementation
   // Not meant to be used publicly unless the implementation is known for sure
@@ -242,10 +244,15 @@ class Color {
    * </div>
    */
   toString(format) {
-    // NOTE: memoize
-    return serialize(this._color, {
-      format
-    });
+    // NOTE: memoize more
+    let colorString = serializationMap.get(this._color);
+    if(!colorString){
+      colorString = serialize(this._color, {
+        format
+      });
+      serializationMap.set(this._color, colorString);
+    }
+    return colorString;
   }
 
   /**