fn from_new_leaf(leaf: Box<LeafNode<K, V>>) -> Self {
        NodeRef { height: 0, node: NonNull::from(Box::leak(leaf)), _marker: PhantomData }
    }
Returns the number of levels that the node and leaves are apart. Zero
height means the node is a leaf itself. If you picture trees with the
root on top, the number says at which elevation the node appears.
If you picture trees with leaves on top, the number says how high
the tree extends above the n
benchmarks/haystacks/code/rust-library.rs