Improved to_string(std::pair) support.

[SVN r79252]

include/boost/exception/to_string.hpp

@@ -19,11 +19,16 @@
 namespace
 boost
     {
+    template <class T,class U>
+    std::string to_string( std::pair<T,U> const & );
+    std::string to_string( std::exception const & );
+
     namespace
     to_string_detail
         {
         template <class T>
         typename disable_if<is_output_streamable<T>,char>::type to_string( T const & );
+        using boost::to_string;
 
         template <class,bool IsOutputStreamable>
         struct has_to_string_impl;