Support a new `{% set %}` tag and defining dicts, list, sets and tuples in `{% with %}` and `{% set %}`

I am one of those remaining adamently opposed to arbitrary extensions of the DTL.

If you need Jinja-like functionality, use Jinja. It’s there for precisely that reason.

For more details about why I hold this opinion, see Django design philosophies DTL - confirm if should update - #9 by KenWhitesell