Add generic overloads for Equatable.== in SetAlgebra types

[lorentey]

It is customary for SetAlgebra types to provide flexible overloads for isSubset/isDisjoint/intersection/etc that can take more types than just Self -- elsewhere we usually have an unconstrained generic overload that can take any Sequence plus a handful of concrete overloads for select types for which we can do a more efficient implementation. (So e.g. OrderedSet provides not only isSubset(of other: Self), but also isSubset(of other: Set<Element>) and isSubset(of other: some Sequence<Element>).)

This applies to all such operations except Equatable.== -- why can't I easily check if a Set and an OrderedSet contain exactly the same members?

Now we wouldn't want to add generic overloads to the == operator, but we can and should add a family of isEqual(to:) methods to types conforming to SetAlgebra that provide this flexibility.

[lorentey]

Closing as implemented. We still need to discuss the new API names, but this PR isn't alone in that.