Moved checked deleters into internal namespace to block unintended ADL.

This prevents adding namespace boost to ADL when the deleters are used
in template parameters, e.g. in std::unique_ptr.

include/boost/core/checked_delete.hpp

@@ -60,6 +60,10 @@ template<class T> inline void checked_array_delete(T * x) BOOST_NOEXCEPT
     delete [] x;
 }
 
+// Block unintended ADL
+namespace checked_deleters
+{
+
 template<class T> struct checked_deleter
 {
     typedef void result_type;
@@ -83,6 +87,11 @@ template<class T> struct checked_array_deleter
     }
 };
 
+} // namespace checked_deleters
+
+using checked_deleters::checked_deleter;
+using checked_deleters::checked_array_deleter;
+
 } // namespace boost
 
 #endif  // #ifndef BOOST_CORE_CHECKED_DELETE_HPP