apex::iter namespace should be used to implement customization point objects for apex::mixin::iterator

[bruxisma]

The advent of CPOs (customization point objects) for C++ dispatching means that apex::mixin::iterator could technically be implemented entirely by way of CPOs. In addition to ranges related traits (e.g., incrementable_traits, we can also rely on iter::next, iter::prev, etc. to see if a type meets the given interface for a specific iterator), having iter:: specific CPOs would allow us to also eventually wrap iterators into type erased iterator APIs. This will also make hooking into ranges easier in the future.

[bruxisma]

Although apex::iter::read_from and apex::iter::write_into have not actually been implemented yet, the rest of the operations are now available. Once read_from and write_into are available, this issue can be closed.