gsl::at overload for initializer_list

initializer_list do not have subscript operator, so the generic container overload of gsl::at fails to compile.
This commits adds an overload of gsl::at for initializer_lists, using *(initializer_list::begin()+index) instead of subscript operator

include/gsl_util.h

@@ -128,6 +128,10 @@ template <class Cont>
 constexpr typename Cont::value_type& at(Cont& cont, size_t index)
 { Expects(index < cont.size()); return cont[index]; }
 
+template <class T>
+constexpr const T& at(std::initializer_list<T> cont, size_t index)
+{ Expects(index < cont.size()); return *(cont.begin() + index); }
+
 } // namespace gsl