New port: devel/microsoft-gsl

The Guidelines Support Library (GSL) contains functions and types that are
suggested for use by the C++ Core Guidelines maintained by the Standard C++
Foundation. This repo contains Microsoft's implementation of GSL.

The library includes types like span<T>, string_span, owner<> and others.

The entire implementation is provided inline in the headers under the gsl
directory. The implementation generally assumes a platform that implements
C++14 support. There are specific workarounds to support MSVC 2015.

WWW: https://github.com/microsoft/GSL

PR: 244551
Submitted by: Henry Hu <henry.hu.sh@gmail.com>