pub fn checksum(&self) -> io::Result<u32> {
        let mut hasher = crc32fast::Hasher::new();
        for kv_res in self.iter() {
            let (k, v) = kv_res?;
            hasher.update(&k);
            hasher.update(&v);
        }
        Ok(hasher.finalize())
    }
Returns the CRC32 of all keys and values
in this Tree.

This is O(N) and locks the underlying tree
for the duration of the entire scan.
src/tree.rs