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

include/boost/container/node_handle.hpp

@@ -123,7 +123,7 @@ class node_handle
 
    void destroy_deallocate_node()
    {
-      nator_traits::destroy(this->node_alloc(), boost::movelib::to_raw_pointer(m_ptr));
+      boost::movelib::to_raw_pointer(m_ptr)->destructor(this->node_alloc());
       nator_traits::deallocate(this->node_alloc(), m_ptr, 1u);
    }