sorted_vector_types use the underlying container value_type

Summary:
Allow the user to give their own pair implementation. This allows
using a packed pair.

Reviewed By: yfeldblum

Differential Revision: D7504521

fbshipit-source-id: 11888594b8b3ef3f530ebd95b8006ef6f5afbc65

folly/sorted_vector_types.h

@@ -566,7 +566,7 @@ class sorted_vector_map
  public:
   typedef Key                                       key_type;
   typedef Value                                     mapped_type;
-  typedef std::pair<key_type,mapped_type>           value_type;
+  typedef typename Container::value_type            value_type;
   typedef Compare                                   key_compare;
 
   struct value_compare : private Compare {