Implement a reverse_iterator, enable rbegin(), rend() etc.

[eyalroz]

We've already added support for some of the <iterator> methods for accessing ranges, like std::begin() and std::end(). But - we haven't added any of their "reverse" variant,s e.g. std::rbegin() and std::rend(). To create kat versions of them, we need the reverse_iterator template, which needs to be brought over from libstdc++'s bits/stl_iterator.h. We already have normal_iterator from there - now let's get the reverse_iterator.