Introduce a new TreeSet struct in sum_tree

This is just a special case of `TreeMap<K, V>` where `V = ()`.

crates/sum_tree/src/sum_tree.rs

@@ -5,7 +5,7 @@ use arrayvec::ArrayVec;
 pub use cursor::{Cursor, FilterCursor, Iter};
 use std::marker::PhantomData;
 use std::{cmp::Ordering, fmt, iter::FromIterator, sync::Arc};
-pub use tree_map::TreeMap;
+pub use tree_map::{TreeMap, TreeSet};
 
 #[cfg(test)]
 const TREE_BASE: usize = 2;