This is my best attempt at fixing the issues raised by @gdr-at-ms while supporting VS2013, VS2015, providing shorter syntax than the static_cast and giving developer meaningful errors when they do the wrong thing.
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)