Implement union, intersection, and difference functions for TrieSet.

[csouth3]

TrieSet doesn't yet have union, intersection, difference, and symmetric difference functions implemented. Luckily, TrieSet is largely similar to TreeSet, so I was able to reference the implementations of these functions in the latter, and adapt them as necessary to make them work for TrieSet.

One thing that I thought was interesting is that the Iterator yielded by iter() for TrieSet iterates over the set's values directly rather than references to the values (whereas I think in most cases I see the Iterator given by iter() iterating over immutable references), so for consistency within TrieSet's interface, all of these Iterators also iterate over the values directly. Let me know if all of these should be instead iterating over references.

[Gankra]

Yes, although we do currently store the integers in their entirety, strictly speaking a Trie doesn't necessarily do that. So yielding keys by value is probably The Right Thing.