Improve powerSet performance

Obviously, there's only so much we can do, but we can do some. The most obvious optimization is to use a version of `insertMin` that takes a singleton tree as an argument instead of an element. But I can't help wondering if we can do better. The problem is obviously $\Omega(2^n)$, and our solution is $O(2^n\log n)$. Can we close that gap by either improving our solution or proving a tight(er) lower bound?