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.
The size of the `src/core/obj.js` file has increased slowly over the years, and it also contains a fair amount of *distinct* functionality.
In order to improve readability and make it easier to navigate through the code, this patch moves the `ObjectLoader` into its own file.
