Split core library from high-level utilities

Libraries like Play Framework usually only need to depend on the low-level library for reading and writing JSON: JsValue, Json.fromJson/Json.stringify, etc. Many of the newer changes in play-json relate to high-level functionality like the macros and configuration that would typically only be used by the end user.

It may be useful to separate this "core" library from the more high-level parts of the library like the macros and functional transformers. This way we can add features and make binary incompatible changes to the macros (for example) without creating compatibility issues with libraries like Play.

Start by mapping the low-level APIs mentioned—JsValue, Json.fromJson, and Json.stringify—against the macros, configuration, and functional transformers described in the issue. Done means there is a clear core/high-level library boundary that lets Play depend on the core without compatibility issues from high-level changes.