Deprecate thrust iterator traversal

[bernhardmgruber]

From my understanding (by reading the Boost.Iterator docs) iterator traversal is distinct from element access, so a proxy iterator (not returning a T&) can also be declared to be random access. E.g. a counting iterator is a input_iterator, because its value type is not a reference. However, this concept is obsolete in C++20, where iterator tags were replaced in favor of iterator concepts.

We should move any thrust iterator traversal trait into a detail namespace and add deprecated public aliases. Users should move away from this functionality.

We should replace iterator traversal by the C++20 iterator concepts in the long run.

[bernhardmgruber]

Some facilities in Thrust query the iterator traversal to select between code paths. Before we can remove this facility we should make sure the C++20 iterator concepts are up and running, for which we do not have enough time for the CCCL 2.8 release.