Updated SketchMap to use SortedSet

closes https://github.com/twitter/algebird/issues/273

There is one quirk that I think is acceptable given how people should be using this, but is still a quirk. The SortedSet that the SketchMap uses can't really have elements added to it. So while the type does more properly encapsulate what is going on, we don't really get to fully leverage SortedSet here.

Unsure how much of a win this is, really, though the types are more explicit.