add %naturalvar to std::string and remove local li_std_string.i, so Mattias can finish the examples here :)

git-svn-id: https://swig.svn.sourceforge.net/svnroot/swig/trunk/SWIG@8635 626c5289-ae23-0410-ae9c-e8d60b6d4f22

Lib/chicken/std_string.i

@@ -6,6 +6,8 @@
 %}
 
 namespace std {
+    %naturalvar string;
+  
 
     %insert(closprefix) %{ (declare (hide <std-string>)) %}
     %nodefault string;