[PATCH 2 of 2 V2] smartset: use native set operations as fast paths

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-02-19 22:43:18 +0900:
> Nit:
> It doesn't preserve the istopo flag. That's okay since '_ascending is not None'
> and _istopo should be mutually exclusive, but I hope it's guaranteed. If you
> construct a baseset from a set with istopo=True, baseset blindly trust it.

I removed it in V2 thinking it was unnecessary. Now I see it's actually
useful - will add it back. Thanks!