Moved shared_container_iterator.hpp in iterator directory, modernized code.

shared_container_iterator now uses std::shared_ptr to store the reference
to the container. boost::shared_ptr is still supported and is wrapped
into std::shared_ptr on construction, so there is overhead due to allocation
of std::shared_ptr state. Going forward, std::shared_ptr is expected
to be the primary use case. As a bonus, this eliminates the dependency
on Boost.SmartPtr.

Moved shared_container_iterator.hpp into the iterator directory and left
a forwarding header for backward compatibility.

doc/index.html

@@ -140,7 +140,7 @@ sequence, rearranged according to some sequence of integer indices.</li>
 bidirectional sequence in reverse.  Corrects many of the
 shortcomings of C++98's <tt class="docutils literal"><span class="pre">std::reverse_iterator</span></tt>.</li>
 <li><a class="reference external" href="../../utility/shared_container_iterator.html"><tt class="docutils literal"><span class="pre">shared_container_iterator</span></tt></a>: an iterator over elements of a container whose
-lifetime is maintained by a <a class="reference external" href="../../smart_ptr/shared_ptr.htm"><tt class="docutils literal"><span class="pre">shared_ptr</span></tt></a> stored in the iterator.</li>
+lifetime is maintained by a <tt class="docutils literal"><span class="pre">shared_ptr</span></tt> stored in the iterator.</li>
 <li><a class="reference external" href="transform_iterator.html"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt></a> (<a class="reference external" href="transform_iterator.pdf">PDF</a>): an iterator over elements which are the result of
 applying some functional transformation to the elements of an
 underlying sequence.  This component also replaces the old