Generalize NLOHMANN_DEFINE_TYPE

[kunaltyagi]

Right now, NLOHMANN_DEFINE_TYPE macro allows only json aka basic_json<> conversions. Would it make sense to:

1. Define the functions on templated basic_json<T> or perhaps JSON with some constraints
2. Specialize for ordered_json because it'd be faster to reserve + emplace_back in to_json and iterate using index + compare key + copy.

I can create a PR if this makes sense.