Add general iteration support in the RefSet and RefSetCache classes

This patch removes the existing `forEach` methods, in favor of making the classes properly iterable instead. Given that the classes are using a `Set` respectively a `Map` internally, implementing this is very easy/efficient and allows us to simplify some existing code.

src/core/evaluator.js

@@ -352,9 +352,9 @@ class PartialEvaluator {
     // When no blend modes exist, there's no need re-fetch/re-parse any of the
     // processed `Ref`s again for subsequent pages. This helps reduce redundant
     // `XRef.fetch` calls for some documents (e.g. issue6961.pdf).
-    processed.forEach(ref => {
+    for (const ref of processed) {
       nonBlendModesSet.put(ref);
-    });
+    }
     return false;
   }