Add an abstract base-class, which all the various Stream implementations inherit from

By having an abstract base-class, it becomes a lot clearer exactly which methods/getters are expected to exist on all Stream instances.
Furthermore, since a number of the methods are *identical* for all Stream implementations, this reduces unnecessary code duplication in the `Stream`, `DecodeStream`, and `ChunkedStream` classes.

For e.g. `gulp mozcentral`, the *built* `pdf.worker.js` files decreases from `1 619 329` to `1 616 115` bytes with this patch-series.

src/core/primitives.js

@@ -14,6 +14,7 @@
  */
 
 import { assert, unreachable } from "../shared/util.js";
+import { BaseStream } from "./base_stream.js";
 
 const EOF = {};
 
@@ -383,7 +384,7 @@ function isRefsEqual(v1, v2) {
 }
 
 function isStream(v) {
-  return typeof v === "object" && v !== null && v.getBytes !== undefined;
+  return v instanceof BaseStream;
 }
 
 function clearPrimitiveCaches() {