Provide std::is_(lt,gt,n,)eq for Q*Ordering types

The std::is_eq family of functions (https://en.cppreference.com/w/cpp/utility/compare/named_comparison_functions) can be considered part of the API of the std::_ordering types. As such, our own ordering types should support these free functions, too, and not just when compiling in C++20 mode.

In order to avoid ambiguities with the std:: functions, these operators should be implemented as hidden friends in each of the three Q*Ordering classes, not as one free function taking QPartialOrdering. That means they will not be candidates when the user writes std::is_eq, and don't cause ambiguities when the user uses unqualified name lookup on std::_ordering types.

Acceptance criteria:

• Each of the three Q*Ordering classes has the full set of six is_ functions as hidden friends
• These functions are documented and tested.