Replace a bunch of Array.prototype.forEach() cases with for...of loops instead

Using `for...of` is a modern and generally much nicer pattern, since it gets rid of unnecessary callback-functions. (In a couple of spots, a "regular" `for` loop had to be used.)

src/core/worker_stream.js

@@ -43,10 +43,9 @@ class PDFWorkerStream {
     if (this._fullRequestReader) {
       this._fullRequestReader.cancel(reason);
     }
-    const readers = this._rangeRequestReaders.slice(0);
-    readers.forEach(function (reader) {
+    for (const reader of this._rangeRequestReaders.slice(0)) {
       reader.cancel(reason);
-    });
+    }
   }
 }