refactor: Backup anchors without hrefs, for compatibility with autorefs' Markdown anchors

[pawamoy]

This will be useful for the soon-to-come Markdown anchors feature of mkdocs-autorefs.

Related to mkdocs-autorefs#39: mkdocstrings/autorefs#39

[pawamoy]

I first went with a custom iterator approach (yielding parent, index, element) to insert at the right position, but since it iterated forward and we had to insert at position or position + 1 it was causing infinite loops.

Instead, I used a recursive approach, and at each layer I iterate on elements backwards to be able to insert in the being-iterated elements. The other option was to iterate on a copy of the list of elements, but this would have been less efficient. reversed is possible here because Element implements both __len__ and __getitem__.

[oprypin]

Looks awesome otherwise :)

[pawamoy]

OK LGTM! Squashing incoming 🙂