[iterators][segment_iterator] add basic dispatch definitions for the
segment_iterator and segment_iterator_type classes
[iterators][segment_iterator] add implementation for an iterator over
the segments of a range
[iterators][segment_iterator] add implementation for the underlying
segment_iterator type
[iterators][segment_iterator] add implementation for the segment_iterator
class, as well as for the free functions segments_begin and segments_end
[test][iterators][segment_iterator] add unit test for the segment_iterator
[test][iterators] add the unit test for segment_iterator in the Jamfile;
re-arrange the unit tests in the iterators' Jamfile in alphabetical ordering;