Introduce sum_tree::TreeMap<K, V>

I think this will be useful to avoid cloning HashMaps in certain cases such as snapshots.

crates/sum_tree/src/sum_tree.rs

@@ -1,9 +1,11 @@
 mod cursor;
+mod tree_map;
 
 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;
 
 #[cfg(test)]
 const TREE_BASE: usize = 2;