pub fn clear(&self) -> io::Result<()> {
        for k in self.iter().keys() {
            let key = k?;
            let _old = self.remove(key)?;
        }
        Ok(())
    }
Clears the `Tree`, removing all values.

Note that this is not atomic.

Beware: performs a full O(n) scan under the hood.
src/tree.rs