Refactor node implemenations from 3 (slist_node, list_node and tree_node) to a single base_node:

- To avoid unneeded includes, is_pair is factored out from pair.hpp implementation
- Fixed bug where value_types inside nodes were not allocator-ware destructed.
- Changed scoped utilities to handle new clases

test/set_test.cpp

@@ -18,6 +18,7 @@
 #include "propagate_allocator_test.hpp"
 #include "emplace_test.hpp"
 #include "../../intrusive/test/iterator_test.hpp"
+#include <utility> //for std::pair
 
 using namespace boost::container;