Unordered: Implement allocator propagation on assignment.

It's pretty messy because I'm trying to avoid swapping allocators in
these cases. I'm also not sure of the exception requirements of
allocator swap and assignment.

[SVN r73756]

include/boost/unordered/detail/node.hpp

@@ -21,6 +21,11 @@
 
 namespace boost { namespace unordered { namespace detail {
 
+    // Some forward declarations for buckets and tables
+    
+    template <typename T> class table;
+    template <class A, bool Unique> class buckets;
+
     ////////////////////////////////////////////////////////////////////////////
     //
     // This section implements buckets and nodes. Here's a rough