Conform to SetAlgebraType for Swift 2

[robrix]

SetAlgebraType is new in Swift 2 and has default implementations.

[natecook1000]

This would be API-breaking, since several of the methods have different names in SetAlgebraType than MultiSet: subtract instead of difference, all the inclusion tests, perhaps more. Any problem going ahead with that?

[robrix]

There might be cases where we’d want to retain the old names, but I doubt it.

I think we can leave the old names in but use the unavailable attribute to indicate their replacements. (It’d be even cooler if we could use that to suggest Fix-its for Xcode’s ingestion, but I doubt that would work.)

Anyway, no problem at all; in fact, I greatly appreciate your initiative!