Add caching to reduce the number of Ref objects

This is similar to the existing caching used to reduced the number of `Cmd` and `Name` objects.
With the `tracemonkey.pdf` file, this patch changes the number of `Ref` objects as follows (in the default viewer):

|          | Loading the first page | Loading *all* the pages |
|----------|------------------------|-------------------------|
| `master` | 332                    | 3265                    |
| `patch`  | 163                    | 996                     |

src/core/primitives.js

@@ -14,6 +14,8 @@
  */
 /* uses XRef */
 
+import { assert } from '../shared/util';
+
 var EOF = {};
 
 var Name = (function NameClosure() {
@@ -176,6 +178,8 @@ var Dict = (function DictClosure() {
 })();
 
 var Ref = (function RefClosure() {
+  const refCache = Object.create(null);
+
   function Ref(num, gen) {
     this.num = num;
     this.gen = gen;
@@ -185,13 +189,19 @@ var Ref = (function RefClosure() {
     toString: function Ref_toString() {
       // This function is hot, so we make the string as compact as possible.
       // |this.gen| is almost always zero, so we treat that case specially.
-      if (this.gen !== 0) {
-        return `${this.num}R${this.gen}`;
+      if (this.gen === 0) {
+        return `${this.num}R`;
       }
-      return `${this.num}R`;
+      return `${this.num}R${this.gen}`;
     },
   };
 
+  Ref.get = function(num, gen) {
+    const key = (gen === 0 ? `${num}R` : `${num}R${gen}`);
+    const refValue = refCache[key];
+    return (refValue ? refValue : (refCache[key] = new Ref(num, gen)));
+  };
+
   return Ref;
 })();
 
@@ -277,6 +287,11 @@ function isRef(v) {
 }
 
 function isRefsEqual(v1, v2) {
+  if (typeof PDFJSDev === 'undefined' ||
+      PDFJSDev.test('!PRODUCTION || TESTING')) {
+    assert(v1 instanceof Ref && v2 instanceof Ref,
+           'isRefsEqual: Both parameters should be `Ref`s.');
+  }
   return v1.num === v2.num && v1.gen === v2.gen;
 }