Consider implementing the <=> for cases unsupported by std::variant

[libbooze]

First of all IDK if there are any spaceship plans for variant2 so this is highly speculative.

Secondly I am not some PL expert, and I do not know all C++ corner cases that might make this impossible, this is purely based on what I consider nicer to use.

In the following example to get std::variant <=> to work we must provide<=>for std::is_empty_v<T> == true kind of struct although it has no state.

#include<type_traits>
#include<variant>

struct Empty{
 // variant <=> does not compile without this line       
 auto operator<=>(const Empty& other) const = default;
};

static_assert(std::is_empty_v<Empty>);

int main() {
    std::variant<Empty,int,float> v1,v2;
    v1<=>v2;
}

If <=> support is added to the variant2 it would be nice if he would be able to skip over empty structs when checking that all elements have implemented <=> (he would treat all instances of empty structs as being std::strong_order::equal).

Convenience motivation for this is that unlike some other languages C++ enums can not carry state( IIRC Rust, Swift... can) so people sometimes use variant of structs as a C++ enum. Now some of those structs are pure "tag" or enum struct without any state, it is a bit of a shame to be required add a <=> to them just so they can be put in variant2.