Simplify `sol::wrap`

[Smertig]

Now sol::wrap is implemented as follows: https://github.com/ThePhD/sol2/blob/a7da2a8e889498b0c5a4ed6d9e3463af02bb6102/include/sol/function_types_templated.hpp#L132-L139

This is not so user-friendly API, because one should use sol::wrap<decltype(<expr>), <expr>> (or write macro to avoid copy-pasting).

I suggest replacing it with:

template <auto F>
struct wrap {
    using type = decltype(F);

    ...
};

• sol2 requires C++17, so there should be no concern about using auto in non-type template parameters
• this is a breaking change, however upcoming v4.0.0 release is the right time to do it
• if you don't want to break anything, you can simply add template <auto F> struct foobar with different name, but sol::wrap is a really good name for such a wrapper

---

P.S. you can also refactor sol::function_detail with auto F feature without breaking sol2 API: https://github.com/ThePhD/sol2/blob/a7da2a8e889498b0c5a4ed6d9e3463af02bb6102/include/sol/function_types_templated.hpp#L30-L115