Unordered: Merge unordered from trunk.

- Activate `std::allocator_traits` for gcc 4.7 and Visual C++ 11.
- Implement variadic construct in `boost::unordered::detail::allocator_traits`
  when variadics, rvalue references and SFINAE expression are available.
- Use variadic construct from `allocator_traits`, or when not available move
  the logic for constructing `value_type` to a lower level, so the container
  code is a bit simpler.
- Avoid `-Wshadow` warnings. Fixes #6190.
- Implement `reserve`. Fixes #6857.


[SVN r78432]

include/boost/unordered/unordered_map.hpp

@@ -515,6 +515,7 @@ namespace unordered
         float load_factor() const;
         void max_load_factor(float);
         void rehash(size_type);
+        void reserve(size_type);
 
 #if !BOOST_WORKAROUND(__BORLANDC__, < 0x0582)
         friend bool operator==<K,T,H,P,A>(
@@ -997,6 +998,7 @@ namespace unordered
         float load_factor() const;
         void max_load_factor(float);
         void rehash(size_type);
+        void reserve(size_type);
 
 #if !BOOST_WORKAROUND(__BORLANDC__, < 0x0582)
         friend bool operator==<K,T,H,P,A>(
@@ -1300,6 +1302,12 @@ namespace unordered
         table_.rehash(n);
     }
 
+    template <class K, class T, class H, class P, class A>
+    void unordered_map<K,T,H,P,A>::reserve(size_type n)
+    {
+        table_.reserve(n);
+    }
+
     template <class K, class T, class H, class P, class A>
     inline bool operator==(
             unordered_map<K,T,H,P,A> const& m1,
@@ -1606,6 +1614,12 @@ namespace unordered
         table_.rehash(n);
     }
 
+    template <class K, class T, class H, class P, class A>
+    void unordered_multimap<K,T,H,P,A>::reserve(size_type n)
+    {
+        table_.reserve(n);
+    }
+
     template <class K, class T, class H, class P, class A>
     inline bool operator==(
             unordered_multimap<K,T,H,P,A> const& m1,