Concept for dealing with larger API changes

Some features such as issue #517 introduce large API changes. At the moment we build all language bindings against the head of the NIX master branch. Therefore changes in the API of the NIX C++ library breaks all bindings.
At the moment this could only be solved by introducing changes to the NIX C++ library and all bindings simultaneously. Although possible, this is hardly practical for larger changes. Therefore we need to come up with a different strategy.

Start by reading issue #517 and examining how the language bindings are currently built against the NIX master branch. Develop and document a strategy that allows larger C++ API changes without requiring simultaneous binding updates; done means the approach is agreed and its compatibility boundaries are clear.

Written by the indexing model from the issue text.