[PATCH 1 of 2] smartset: convert set to list lazily

[Jun Wu]

Excerpts from Martin von Zweigbergk's message of 2017-02-18 17:03:12 -0800:
> @@ -205,5 +215,5 @@ class baseset(abstractsmartset):
> 
>      def __nonzero__(self):
> -        return bool(self._list)
> +        return bool(len(self))
> 
> Could delete method, I think. Just FYI.

No, because abstractsmartset implements __nonzero__ to
"raise NotImplementedError()".

I think that's reasonable as it will force people to not forget overriding
the __nonzero__ implementation, to make things more explicit.