Deprecate Standalone for 1.78

[vinniefalco]

When standalone is defined we need to generate a warning message for 1.78. Then in 1.79 we will remove it (after forking it to a new repository on https://github.com/CPPAlliance)

[doganulus]

How would you continue to develop then? Develop on standalone continuously and integrate/snapshot on Boost releases?

[vinniefalco]

Well they would diverge. Some features might be ported but it is doubtful, because Boost offers facilities that are not available in C++17. For example, mp11. And Describe. A better error_code with source-file information. And other things. Keeping standalone features on parity with Boost.JSON means that the library cannot improve as much.

We are working on making string_view and error_code compatible with their C++17 counterparts so that using the Boost-flavored JSON in C++17 or later is a more integrated experience.