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

This reverts commit 44cee00831.

The commit reportedly broke building Boost and quickbook.

https://github.com/boostorg/iterator/commit/b844c8df530c474ec1856870b9b0de5f487b84d4#commitcomment-30603668

include/boost/iterator/advance.hpp

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

include/boost/iterator/distance.hpp

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