Fix issue #314: Added json_fwd.hpp.

[ralphtandetzky]

The issue asks: "Is there a way to forward declare nlohmann::json?"

This commits helps to do that. The added file helps to reduce compilation dependencies by providing a forward declaration of nlohmann::basic_json and a typedef for nlohmann::json.

[coveralls]

Coverage remained the same at 99.904% when pulling a8af911 on ralphtandetzky:ralphtandetzky-json_fwd into a0ef5a1 on nlohmann:develop.

[theodelrieu]

Hi, sorry about the delay since this PR was opened, but due to 2.1.0 changes, could you update the forward declarations to have adl_serializer in it?

Also, if you could provide a simple test file, I think @nlohmann will be able to close this PR ;)

[nlohmann]

I still need to check for myself whether the compilation times are really that much better.

[theodelrieu]

Sure, I guess it became huger since 2.1.0, with all the from/to_json prototypes in users' headers. But I didn't make any measurement, this is just an assumption

[zetaPRIME]

Here's a version of json_fwd compatible with 2.1.1 if anyone needs one.
https://github.com/zetaPRIME/libstarlight/blob/edf7945acdf567c9b7e65923daf79aee5cc8fc69/libstarlight/source/starlight/_incLib/json_fwd.hpp

[AraHaan]

I would really like something like this to get merged for when people need to use things like this.

[theodelrieu]

This is something which is likely to be added in 3.0.0

As I proposed in #474 , I'd like to have separate headers for development.
We will continue to release the single header json.hpp, but I think we should also release the separate headers version for those who want it.

[nlohmann]

This won't be merged in any time soon, so I close the PR. Let's discuss future steps in #474.