[boost] [iterator] Move iterators to a separate namespace

Hi, I've created this pull request a few days ago: https://github.com/boostorg/iterator/pull/5 It basically moves all iterators to boost::iterators namespace (while still importing the symbols to boost namespace for backward compatibility). This change is also intended to fix the counting_iterator_test test on older compilers, which was broken by my previous pull request (there appeared a name clash with Boost.TypeTraits). Could it be merged?