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.

build.jam

@@ -14,7 +14,6 @@ constant boost_dependencies :
     /boost/fusion//boost_fusion
     /boost/mpl//boost_mpl
     /boost/optional//boost_optional
-    /boost/smart_ptr//boost_smart_ptr
     /boost/type_traits//boost_type_traits
     /boost/utility//boost_utility ;