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/node_handle_test.cpp

@@ -140,15 +140,18 @@ struct node
       ++count;
    }
 
-   ~node()
-   {
-      --count;
-   }
+   template<class Alloc>
+   void destructor(Alloc &)
+   {  this->~node(); }
 
    static unsigned int count;
 
    static void reset_count()
    {  count = 0;  }
+
+   ~node()
+   {  --count; }
+
 };
 
 template<class T1, class T2>