Remove `comparator.hh` and use new C++ comparison operator defaulting

[Ericson2314]

See the docs on Default comparisons (since C++20) https://en.cppreference.com/w/cpp/language/default_comparisons

In short, one can now do

bool operator == (const MyType & h2) const = default;
auto operator <=> (const MyType & h2) const = default;

And this will work in the vast majority of cases.

This is better than the src/libutil/comparator.hh tricks we did before (predating C++ 20) of deriving those methods based on constructing a std::tuple with all the fields we wanted to compare on the fly.

[Ericson2314]

After https://github.com/NixOS/nix/pull/10838, just have a couple of GENERATE_CMP left. However, the GENERATE_CMP_EXT We'll need to keep for now. That's OK, though.