Motivated by `std::string_view` being usable in
const (compile-time) context.
One functional change was needed for StringRef:
`std::char_traits<char>::length(str)` instead of `strlen`.
Reviewed By: JacquesLucke, LazyDodo
Differential Revision: https://developer.blender.org/D9788
Since C++17 there is also std::string_view, which is similar to StringRef.
This commit does a couple of things:
* New implicit conversions between StringRef and std::string_view.
* Support std::string_view in blender::DefaultHash.
* Support most of the methods that std::string_view has.
* Add StringRef::not_found which can be used instead of -1 in some places.
* Improve/fix the description at the top of BLI_string_ref.hh.
