Fix magic_enum::containers::set erase function for Clang compilers

[Ry-DS]

The erase function does not compile on the following:

clang version 10.0.0-4ubuntu1 
Target: x86_64-pc-linux-gnu
Thread model: posix

When building for C++ 17 and 20 (note these specific configurations are all I tested).

Compile error:

include/magic_enum_containers.hpp:985:10: error: no viable conversion from 'typename container_type::reference' (aka 'reference_impl<>') to 'bool'
    bool res = ref;
         ^     ~~~
(retracted): note: in instantiation of member function 'magic_enum::containers::set<Retracted>::erase' requested here
        set.erase(Retracted::ELEMENT);
                         ^
include/magic_enum_containers.hpp:526:38: note: explicit conversion function is not a candidate
    [[nodiscard]] constexpr explicit operator bool() const noexcept { return (parent->a[num_index] & bit_index) > 0; }

This PR fixes that, while adding tests for this function to ensure this template is instansiated and exercised.

[Neargye]

Thanks!

[Ry-DS]

In hindsight, removing the explicit keyword here:

magic_enum/include/magic_enum_containers.hpp

Line 535 in fd9f70b

[[nodiscard]] constexpr explicit operator bool() const noexcept { return (parent->a[num_index] & bit_index) > 0; }

might have made more sense. Regardless either solution works, thanks for the prompt merge + release! 🙌

[Neargye]

Yes, perhaps this fix makes sense, in the original bitset it is not marked explicit https://en.cppreference.com/w/cpp/utility/bitset/reference

If you have time, I will be glad to accept PR

[Ry-DS]

was about to make one but I see it's handled in 677636e, thanks!