pub fn len(&self) -> io::Result<usize> {
        let mut count = 0;
        for item_res in self.iter() {
            let _item = item_res?;
            count += 1;
        }
        Ok(count)
    }
Returns the number of elements in this tree.

Beware: performs a full O(n) scan under the hood.

# Examples

```
# fn main() -> Result<(), Box<dyn std::error::Error>> {
# let config = sled::Config::tmp().unwrap();
# let db: sled::Db<1024> = config.open()?;
db.insert(b"a", vec![0]);
db.insert(b"b", vec![1]);
assert_eq!(db.len().unwrap(), 2);
# Ok(()) }
```
src/tree.rs