Accommodate the latest changes in Boost.Iterator.

In pull request https://github.com/boostorg/iterator/pull/5 most implementation details were moved to the boost::iterators and boost::iterators::detail namespaces. This pull request updates Boost.Range to reflect these changes.

The pull request partially depends on another pull request https://github.com/boostorg/iterator/pull/6 which makes pure_traversal_tag a public component and not an implementation detail.