feat: add render_indexed_content_section for indexes feature

[Finistere]

Hi! Currently, implementing render_section with indexes feature would require writing an equivalent to IndexedBasedReader.
As the logic is likely to be the same all the time, I have added a render_indexed_content_section to handle this.

I'm not really sure if that's the best approach though.

Furthermore there was an inconsistency, except Vec<T>, all other list types like &[T] would simply ignore the indexes feature.

[maciejhirsz]

This looks like it should be or remain to be an implementation detail, but I'll need to look back at the code to give a better suggestion.

[Finistere]

An alternative could be to not expose this function publicly but keep the fixes on &[T] etc. and I copy the relevant code.

It feels a bit weird that I would need to implement an IndexedBasedReader myself, seems like something that ramhorns should manage somehow. Maybe Content should have something like a render_list_section(...) -> impl ExactSizeIterator instead of render_index_section(). This would keep Indexed, Index and IndexedBasedReader as implementation details.

But I haven't worked enough with ramhorns for now to be sure. 🤷

[Finistere]

I'm not sure if the indexing capabilities are that relevant to be honest. I'm mostly interested in {{-last}} to make formatting of JSON lists possible. So could be render_list_section(...) -> impl Iterator if only first & last are kept.

[maciejhirsz]

Maybe Content should have something like a render_list_section(...) -> impl ExactSizeIterator instead of render_index_section(). This would keep Indexed, Index and IndexedBasedReader as implementation details.

This seems sensible to me. Give me a day to dig into it first. In the mean time having the helper function but it not being public as a fix for slices is also fine.

[Finistere]

Nothing urgent on my side, take your time! I can keep using our fork for a while. I just hope to not keep it forever, but seems like that won't be the case. :)