Operators for sets and topology

[yudukikun5120]

I wrote operations for sets and topology:

• Set.power_set: returns a power set of a given set
• Set.intersection: returns an intersection of given sets, rather than two sets
• Set.union: returns a union of given sets, rather than two sets
• Topology.topology: returns a topology of a given set

These modules are in my repository (https://github.com/yudukikun5120/topology). But I'd rather merge in your repository considering the size of my package.
So could you consider merging the functions above? If you are willing to do so, I will open the pull request.

Thank you.

[Qqwy]

Since the rest of this library does not deal with sets, I think it is better to keep your library separate.