detail::fast_iterator might satisfy ont of the iterator concepts from the STL

[Simon-Lopez]

Dear all,

With a few additional lines the detail::fast_iterator could satisfy std::forward_iterator concept.
(cf. a proposal commit here).

This would then make for example nanobind::list satisfying the std::ranges::range concept and we could use on nanobind::list all the goodies that come with std::ranges (or range-v3).

More explicitely the following code fails on my machine (gcc 11.3) without the proposed modifications:

#include <iterator>

#include <nanobind/nanobind.h>

NB_MODULE(check, module) {

static_assert(std::forward_iterator<nanobind::detail::fast_iterator>);

}

[wjakob]

This commit was merged in the meantime, thanks for the PR.