Add a C++11 / C++14 std::string_view

[wolfv]

This would allow us to enable some cool features on xtensor<char[20]> arrays and the like!

[wolfv]

maybe this one is good: https://github.com/martinmoene/string-view-lite

[wolfv]

Turns out that string_view is not needed / not the right tool for what I had in mind – adding a storage option to xtl's fixed_string is a much nicer idea.