YAXT

The intention is to have only one header for users which in turn includes a set of headers placed in src/xt. This header shall be named yaxt.h and the current yaxt.h renamed xt_core.h.

A way to produce this header automatically shall be devised. It's currently unclear how to build a workable sequence of inclusion.

The tests shall in the future only use this header to prevent non-public interfaces from being used in the tests.