Add fromIterable and toNativeSet

[gabejohnson]

This would make it easy to convert from/to native types. fromIterable would cover any instance w/ Symbol.iterator on it.

toNativeSet would be a convenience function nearly identical to toArray.

Another option would be to make the set types iterable. Then you could just do:

const nativeDifference = fromNative => toNative => a => pipe([
  fromNative, curry2(difference)(fromNative(a)), toNative]);

const nativeDifference2 = toNative => a => pipe([
  BaseSet.fromIterable, curry2(difference)(BaseSet.fromIterable(a), toNative]);

const arrayDifference = nativeDifference(BaseSet.fromFoldable)(BaseSet.toArray);

const setDifference = nativeDifference(
  compose(BaseSet.fromFoldable, Array.from))(s => new Set(BaseSet.toArray(s)));

const arrayDifference2 = nativeDifference2(Array.from);

const setDifference2 = nativeDifference2(s => new Set(s));

[ottonascarella]

Leveraging Symbol.iterator as a default to deal with lists would probably be the best way for polymorphic reasons...even ImmutableJS uses them...

I mean: all data types that implement list-like should be implemented using iterator.