Robin DaviesSnapshot 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.