CxxUtils: Add span and reverse_wrapper.
Add CxxUtils::span, similar to the C++20 std::span.
Also add reverse_wrapper, an adapter to allow iterating over something in the reverse direction in a range-for.
Add CxxUtils::span, similar to the C++20 std::span.
Also add reverse_wrapper, an adapter to allow iterating over something in the reverse direction in a range-for.