whether using it from the development folder, from the installation
folder or from being copied into a project.
#include <gsl/gsl.h>
Updated headers/tests/instructions/cmake build accordingly
This PR should address https://github.com/Microsoft/GSL/issues/277 (less
the renaming of gsl itself)
Also removed unused constant member variable that seemed to be there
to prevent maybe_null_* being used with anything other than a pointer,
which is being taken care of with a static_assert now.