fn from_internal(node: NonNull<InternalNode<K, V>>, height: usize) -> Self {
        debug_assert!(height > 0);
        NodeRef { height, node: node.cast(), _marker: PhantomData }
    }
Finds the parent of the current node. Returns `Ok(handle)` if the current
node actually has a parent, where `handle` points to the edge of the parent
benchmarks/haystacks/code/rust-library.rs