CxxUtils: Mark that CxxUtils::span is a view. Add at().
For C++20, CxxUtils::span should derive from view_base in order for it to be usable as a view with the ranges library. Also, add at() methods. (These are C++26.)
For C++20, CxxUtils::span should derive from view_base in order for it to be usable as a view with the ranges library. Also, add at() methods. (These are C++26.)