Add a shorthand to masquerade a type parsing/serializing

Start by reading the existing tag_invoke, value_to, value_from, and described-struct conversion entry points and tests. Compare the proposed JsonAs shorthand with the enum, invariant-wrapper, and string-convertible examples; done requires an agreed public API and coverage for the supported conversions.

Hi, there are two cases where I couldn't avoid implementing one-line tag_invokes. I'd love boost::json to do it for me!

The first one is an enum that only has ints. Its an enum class because why not but I don't care about specific values, its just here to help referencing interesting ones.

enum class Version { _202201 = 202201, _202301 = 202301, _202302 = 202302};

Version tag_invoke(boost::json::value_to_tag<Version>, const boost::json::value& json) {
    return static_cast<Version>(json.as_int64());
}
void tag_invoke(boost::json::value_from_tag, boost::json::value& json, Version a) {
    json = static_cast<int>(a);
}

Second case is probably more interesting, following a lot of back and forth the best way I found to handle a type with invariants is to make a described struct.

struct MyClass {
    struct Params {
        ...
    };

    MyClass(Params params);
    ...
};

And MyClass c = value_to<MyClass::Params>() just works (with implicit conversion), but if I want to include it inside a described struct boost::json doesn't know that to decode a MyClass it needs a MyClass::Params.

I also had the same problem when I tried to parse/serialize a type implicitly convertible to and from a std::string. Even overriding is_string_like<> was not enough.

So I made a little helper and it works for me, but I think boost::json should have something similar (or better!) built-in:

template <typename>
struct JsonAs;


namespace boost::json {

template <typename T>
requires requires { typename JsonAs<T>::type; }
T tag_invoke(boost::json::value_to_tag<T>, const boost::json::value& json) {
    return static_cast<T>(boost::json::value_to<typename JsonAs<T>::type>(json));
}

template <typename T>
requires requires { typename JsonAs<std::remove_cvref_t<T>>::type; }
void tag_invoke(boost::json::value_from_tag, boost::json::value& json, T&& a) {
    json = boost::json::value_from(static_cast<JsonAs<std::remove_cvref_t<T>>::type>(std::forward<T>(a)));
}

}

template <>
struct JsonAs<Version> : std::type_identity<int> {};