C++20 proxy iterators most likely don't work

[Morwenn]

Proxy iterators support in cpp-sort was added long before C++20 was a thing, and was added to solve a few internal problems after reading Eric Niebler's articles from back then. A few days ago, I learnt the hard way that I didn't read those article correctly, and as a result one of the main reasons why they were introduced, namely std::views::zip probably does not work at all in cpp-sort.

Now they're a tricky beast because the library's version of proxy iterators also seems slightly more flexible in some regards. I think I can get the best of both worlds, but that requires some experimentation. If I can't fully rely on the standard design, I will still need to support it, but also support the cpp-sort extensions on top of them.

To design and experiment later.