`<ranges>`: iterator and sentinel of `iota_view` should be ADL-proof

Describe the bug
[range.iota] describes the iterator and sentinel types of iota_view as nested classes, but MSVC STL doesn't follow the wording. Difference from the standard wording is observable via ADL (Godbolt link).

#include <ranges>

constexpr int foo(auto) noexcept { return 42; } // selected by libc++ & libstdc++

namespace fvs {
    constexpr int foo(std::input_iterator auto) noexcept { return 0; } // selected by MSVC STL
    struct bar {};
}

using ioit = std::ranges::iterator_t<std::ranges::iota_view<fvs::bar*, fvs::bar*>>;
static_assert(foo(ioit{}) == 42);

I think we should make them non-template classes nested in class templates. IIUC the enclosing class is not nessarily iota_view, which allows SCARY implementation strategies.

See also

• WG21-P2538, and
• How hana::type<T> “disables ADL”.

Start at the implementation of iota_view's iterator and sentinel types and compare it with the [[range.iota]] wording. Use the provided Godbolt reproducer to check the ADL difference; done means the implementation's iterator and sentinel types have the intended ADL-proof behavior.