Support containers with const value type

Currently just storing the value without a const. Can do better with
C++11 constructors, so maybe should do that, and cast away const on
compilers without support.

Another problem is that std::allocator<const int> doesn't compile for
libstdc++ (and potentially other standard libraries), so
boost::unordered_set<const int> can't compile. I'm not sure if I should
work around that, as it means changing the type of the container
(i.e. to boost::unordered_set<const int,... , std::allocator<int>>).

include/boost/unordered/detail/equivalent.hpp

@@ -110,9 +110,11 @@ namespace boost { namespace unordered { namespace detail {
     template <typename A, typename T>
     struct pick_grouped_node
     {
+        typedef typename boost::remove_const<T>::type nonconst;
+
         typedef boost::unordered::detail::allocator_traits<
             typename boost::unordered::detail::rebind_wrap<A,
-                boost::unordered::detail::grouped_ptr_node<T> >::type
+                boost::unordered::detail::grouped_ptr_node<nonconst> >::type
         > tentative_node_traits;
 
         typedef boost::unordered::detail::allocator_traits<
@@ -120,7 +122,7 @@ namespace boost { namespace unordered { namespace detail {
                 boost::unordered::detail::ptr_bucket >::type
         > tentative_bucket_traits;
 
-        typedef pick_grouped_node2<A, T,
+        typedef pick_grouped_node2<A, nonconst,
             typename tentative_node_traits::pointer,
             typename tentative_bucket_traits::pointer> pick;