add support for std::span

I'm currently thinking about a dynamic interface for the communication pattern in dune-istl. For that i would like to have a datatype like std::span, which is proposed for the C++20 standard. I found this repository on GitHub: https://github.com/tcbrindle/span which is licensed under the permissive BSL-1.0 license, that allows (as far as I know) to distribute parts of the code under GPL.

Do you think its a good idea to include that into the Dune::Std namespace like we did for std::optional and std::variant?

If so, I could prepare a merge request.