- Implement the `operator==` and the `operator<=>` (aka the spaceship
operator) in `base_uint`, `Issue`, and `Book`.
- C++20-compliant compilers automatically provide the remaining
comparison operators (e.g. `operator<`, `operator<=`, ...).
- Remove the function compare() because it is no longer needed.
- Maintain the same semantics as the existing code.
- Add some unit tests to gain further confidence.
- Fix#2525.