Some generic algorithms from the standard library like `std::any_of`
did not work with all container and iterator types. To improve the
situation, this patch adds various type members to containers
and iterators.
Custom iterators for Set, Map and IndexRange now have an iterator
category, which soe algorithms require. IndexRange could become
a random access iterator, but adding all the missing methods can
be done when it is necessary.
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
