Implement, test and document "key in dict" and "key not in dict".

Implement, test and document "key in dict" and "key not in dict".

I know some people don't like this -- if it's really controversial,
I'll take it out again.  (If it's only Alex Martelli who doesn't like
it, that doesn't count as "real controversial" though. :-)

That's why this is a separate checkin from the iterators stuff I'm
about to check in next.