string: don't use const members for tokeniser::iterator

this allows us to provide an assignment operator more easily

string.hpp

@@ -116,7 +116,7 @@ namespace util {
                 m_end (_range.cend ())
             { ;  }
 
-            const value_type m_separator;
+            value_type m_separator;
             range_type m_range;
             Iterator m_end;