Update for compatibility with C++20 iterator concepts

[Lastique]

In C++20, forward iterators (and more advanced) no longer require the iterator reference type to be an actual reference. Instead, it is required that a reference can be formed to the reference type.

Since Boost.Iterator iterators still define their categories using C++03 rules, some iterators receive unnecessarily strict categories in terms of C++20. Here is one example.

Perhaps, we should use C++20 rules for deducing iterator categories in C++20 and later and the current rules in C++17 and older.