Auto-detect non-assignable classes

Classes with references or const data are now marked as 'noassign'.
This renders many explicit `private: operator=` declarations redundant.

Examples/test-suite/apply_signed_char.i

@@ -35,7 +35,5 @@
     const char memberconstchar;
 
     virtual ~DirectorTest() {}
-  private:
-    DirectorTest& operator=(const DirectorTest &);
   };
 %}