Drop `Origin` from public API.

We don't really need to expose `Origin` publically. For the sake of keeping the public API space slim we should...

* Drop the `url.origin` property.
* Drop `Origin` from the top-level export.
* Move `Origin` into `utils.py`.
* Use `origin = Origin(url)` in our internal usage, rather than `url.origin`.

Minor breaking change here, although I'm not sure we *document* origin anywhere or not.