SimpleString::StrNCmp()

Okay... something a bit more straightforward first...
This is the right pull request to comment. Bah, sorry for making a mess of my comments.
Changes suggested:
- Change the do-while in a while (or to a for loop and check it from start to finish instead of backwards)
- Remove the if n == 0 (now covered by the while)
- Change the return (0) to return 0;
- Remove the ++ and -- out of the comparisons and put them in the end (for clarity)
- Change the break in a return (?)