Feature: Return traversable

This pull request allows returning traversable structures, see #1.

* [x] Support `php.Traversable` in XML and JSON serializers
* [ ] Optimize case and stream JSON (using new `text.json` APIs?)
* [ ] Optimize case and stream XML (using XML writer?)
This could be further improved by changing `serialize()` not to have to return a string but to be able to write directly to the output stream,