Add experimental small_vector class

include/boost/container/container_fwd.hpp

@@ -98,6 +98,10 @@ class stable_vector;
 template <class T, std::size_t Capacity>
 class static_vector;
 
+template < class T, std::size_t N
+         , class Allocator= new_allocator<T> >
+class small_vector;
+
 template <class T
          ,class Allocator = new_allocator<T> >
 class deque;