Add use_future_::rebind() as replacement for operator[].

asio/include/asio/use_future.hpp

diff --git a/asio/include/asio/use_future.hpp b/asio/include/asio/use_future.hpp
index 1428bcc..15136a8 100644
--- a/asio/include/asio/use_future.hpp
+++ b/asio/include/asio/use_future.hpp
@@ -56,12 +56,21 @@
   {
   }
 
-  /// Specify an alternate allocator.
+#if !defined(ASIO_NO_DEPRECATED)
+  /// (Deprecated: Use rebind().) Specify an alternate allocator.
   template <typename OtherAllocator>
   use_future_t<OtherAllocator> operator[](const OtherAllocator& allocator) const
   {
     return use_future_t<OtherAllocator>(allocator);
   }
+#endif // !defined(ASIO_NO_DEPRECATED)
+
+  /// Specify an alternate allocator.
+  template <typename OtherAllocator>
+  use_future_t<OtherAllocator> rebind(const OtherAllocator& allocator) const
+  {
+    return use_future_t<OtherAllocator>(allocator);
+  }
 
   /// Obtain allocator.
   allocator_type get_allocator() const