Also fixes two long-standing bugs (present in 2.0):
1. .join() didn't check that the result size fit in an int.
2. string.join(s) when len(s)==1 returned s[0] regardless of s[0]'s
   type; e.g., "".join([3]) returned 3 (overly optimistic optimization).
I resisted a keen temptation to make .join() apply str() automagically.