Key and value iterators show their contents on the REPL

@simonster suggested this idea. Since showing a key iterator on the REPL is currently dumping the whole dictionary using the most generic ``show`` routine, it seems we might as well show just the dictionary keys. A frequent complaint about Python 3 is that its inconvenient to inspect dictionaries on the REPL since you constantly forced to collect the key iterators, so why not avoid that.