pub fn get_mut(&mut self, key: &Nibbles) -> Option<&mut Node> {
        let level = self.changes_by_nibble_count.get_mut(key.nibble_count())?;
        let node = level.get_mut(key.bytes())?;
        Some(&mut node.inner)
    }
Marks the retrieved node as changed.
core/lib/merkle_tree/src/storage/patch.rs