Opts strong typing and easier use

[stephenberry]

Discussed in #816

^{Originally posted by arturbac March 9, 2024}
It should work something like this

enum struct format {   json,   csv };
enum struct comments : bool {   no,   yes };


//---------------------- C++20 Version ----------------------

struct opts_cxx20 
   {
    format format_{format::json};
    comments comments_{};
    error_on_unknown_keys error_on_unknown_keys_{};
    
    constexpr opts_cxx20() = default;

    template<typename... Args>
    constexpr opts_cxx20(Args... args) {    (set(args), ...);   }

private:
    constexpr void set(format f) {   format_ = f;  }
    constexpr void set(comments c) {  comments_ = c;  }
    constexpr void set(error_on_unknown_keys e) {   error_on_unknown_keys_ = e;   }
   };

//---------------------- C++17 Version ----------------------

struct opts_cxx17 
   {
    format format_{format::json};
    comments comments_{comments::no};
    error_on_unknown_keys error_on_unknown_keys_{};
    
    constexpr opts_cxx17() = default;

    template<typename... Args>
    constexpr opts_cxx17(Args... args) {  set(args...);  }

private:
    void set() {}

    template<typename T, typename... Args>
    constexpr void set(T first, Args... args) {
        if constexpr (std::is_same_v<std::decay_t<T>, format>) {
            format_ = first;
        } else if constexpr (std::is_same_v<std::decay_t<T>, comments>) {
            comments_ = first;
        } else if constexpr (std::is_same_v<std::decay_t<T>, error_on_unknown_keys>) {
            error_on_unknown_keys_ = first;
        }
        set(args...); // recurse with remaining arguments
    }
};

//---------------------- Example Usage ----------------------

    glz::read_as<opts_cxx20{}>();
    glz::read_as<opts_cxx17{}>();

    glz::read_as<opts_cxx20{format::json}>();
    glz::read_as<opts_cxx17{format::json}>();

    glz::read_as<opts_cxx20{comments::yes}>();
    glz::read_as<opts_cxx17{comments::yes}>();

    glz::read_as<opts_cxx20{error_on_unknown_keys::yes}>(); 
    glz::read_as<opts_cxx17{error_on_unknown_keys::yes}>(); 
    
    glz::read_as<opts_cxx20{format::csv, comments::yes}>();
    glz::read_as<opts_cxx17{format::csv, comments::yes}>();

    glz::read_as<opts_cxx20{comments::no, format::csv}>();
    glz::read_as<opts_cxx17{comments::no, format::csv}>();
    
    glz::read_as<opts_cxx20{format::json, comments::yes, error_on_unknown_keys::yes}>(); 
    glz::read_as<opts_cxx17{format::json, comments::yes, error_on_unknown_keys::yes}>(); 

    glz::read_as<opts_cxx20{error_on_unknown_keys::no, format::csv, comments::no}>(); 
    glz::read_as<opts_cxx17{error_on_unknown_keys::no, format::csv, comments::no}>(); 

 // etc any combination ..
```</div>

[stephenberry]

A downside of this approach is that compilers like to use the enumeration in the template name. The longer the template name, the longer the compile time. So, this change would likely increase compile times.

If we use bit packing internally then we can reduce compile times and make this useful. However, we really need bit packing and unpacking to facilitate handling options, and it makes things annoying having to unpack everywhere, and I'm afraid that this might increase compilation times.