Perform some caching in the reference implementation

In general we are loathe to introduce this kind of departure from the spec. However, it improves our ability to run the tests quickly by a very large factor. See the context at #488 (comment).

reference-implementation/lib/queue-with-sizes.js

@@ -5,6 +5,9 @@ const { IsFiniteNonNegativeNumber } = require('./helpers.js');
 exports.DequeueValue = queue => {
   assert(queue.length > 0, 'Spec-level failure: should never dequeue from an empty queue.');
   const pair = queue.shift();
+
+  queue._totalSize -= pair.size;
+
   return pair.value;
 };
 
@@ -15,19 +18,19 @@ exports.EnqueueValueWithSize = (queue, value, size) => {
   }
 
   queue.push({ value: value, size: size });
+
+  if (queue._totalSize === undefined) {
+    queue._totalSize = 0;
+  }
+  queue._totalSize += size;
 };
 
+// This implementation is not per-spec. Total size is cached for speed.
 exports.GetTotalQueueSize = queue => {
-  let totalSize = 0;
-
-  queue.forEach(pair => {
-    assert(typeof pair.size === 'number' && !Number.isNaN(pair.size) &&
-      pair.size !== +Infinity && pair.size !== -Infinity,
-      'Spec-level failure: should never find an invalid size in the queue.');
-    totalSize += pair.size;
-  });
-
-  return totalSize;
+  if (queue._totalSize === undefined) {
+    queue._totalSize = 0;
+  }
+  return queue._totalSize;
 };
 
 exports.PeekQueueValue = queue => {