Snapshot of https://github.com/microsoft/GSL taken on Feb 9, 2022, implementing

    The C++ Core Guidelines

    https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines

A grossly incovenient nameclash with libgsl = Gnu Scientific Library, along with 
inconvenient Microsoft packaging make it easier just to copy the GSL verbatim.

There's an implicit assumption that the GSL is irrevocably on a C++ standards track.

If and when that happens, just delete this ENTIRE directory.