Better support for non-default-constructable datatypes

[Ericson2314]

I have read the docs on how this library does in fact support deserializing to non-default-constructable types. The problem is that it's not very compositional: for example, there is no way to write a generic instance for std::variant<Ts..> that works if all of Ts.. aren't default construable. Even std::option<T>, which is itself default construable no matter if T is, would be a bit hard to write in an exception-safe way.

In my own code (https://github.com/NixOS/nix/pull/3859/files#diff-48ed85ec1c3c2412447bedb0e859275a if anyone is curious) I tried a different technique to get around this, which adopted to this library would look something like

/* To guide overloading */
template<typename T>
struct Proxy {};

V from_json(const json& j, Proxy<V> _);

the idea is that Proxy guides overloading so we can effectively overload the return type.

[nlohmann]

Maybe related: #2576

[Ericson2314]

The ADL from_json indeed works quite nicely, and we've been using it!