pub unsafe fn next_back_unchecked(&mut self) -> (&'a K, &'a V) {
        super::mem::replace(self, |leaf_edge| {
            let kv = leaf_edge.next_back_kv();
            let kv = unsafe { kv.ok().unwrap_unchecked() };
            (kv.next_back_leaf_edge(), kv.into_kv())
        })
    }
Moves the leaf edge handle to the previous leaf edge and returns the key and value
in between, deallocating any node left behind while leaving the corresponding
edge in its parent node dangling.
benchmarks/haystacks/code/rust-library.rs