Replace using-declarations with using-directives to avoid ADL issues.

include/boost/iterator/advance.hpp

@@ -77,7 +77,7 @@ namespace iterators {
 
 } // namespace iterators
 
-using iterators::advance;
+using namespace iterators::advance_adl_barrier;
 
 } // namespace boost
 

include/boost/iterator/distance.hpp

@@ -58,7 +58,7 @@ namespace iterators {
 
 } // namespace iterators
 
-using iterators::distance;
+using namespace iterators::distance_adl_barrier;
 
 } // namespace boost