forked from microsoft/GSL
* add gsl::not_null operator<< Hippomocks, which is a popular mocking engine for C++ uses operator<< on pointers and gets confused about gsl::not_null not having this operator. * Update gsl
* add gsl::not_null operator<< Hippomocks, which is a popular mocking engine for C++ uses operator<< on pointers and gets confused about gsl::not_null not having this operator. * Update gsl