span being a view and not a container, the generic version of gsl::at is not valid any more for span.
This commits adds a specialization of gsl::at for span
Not all platforms have support for strnlen (e.g. cygwin) as this is a non-standard function. This patch provides a custom implementation of strnlen for these platforms.
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)
