Added substring capability to ZString.

testing/zstring_test.cpp

@@ -22,4 +22,9 @@ int main(int argc, char **argv) {
    for(int ix = 0; ix < test.getList().size(); ++ix) 
      std::cout << ix << ":" << test[ix] << std::endl;
 
+   // Test substring operations.
+
+   coreutils::ZString test2("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+   std::cout << "substring: [" << test2.substring(10) << "]" << std::endl;
+
 }