Modernize comparison operators

This did hit me like a truck.

If you define operator ==, then you have also operator != for free, there is absolutely no need to define it.

I'd suggest the following to modernize our code:

1. If possible, define the three-way comparison R operator<=>(const T& a, const U& b), and remove any other comparison operator.
2. If ordering is not possible (no "greater" or "lesser" definition), only define bool operator==(const T& a, const U& b);. Remove any other comparison operator.
3. Clarify this point in the coding guidelines.

Note that the 3-way comparison requires one to decide if the ordering is strong, partial, or weak, see https://en.cppreference.com/w/cpp/header/compare.html