pub fn left_edge(self) -> Handle<NodeRef<BorrowType, K, V, NodeType>, marker::Edge> {
        unsafe { Handle::new_edge(self.node, self.idx) }
    }
Given an edge index where we want to insert into a node filled to capacity,
computes a sensible KV index of a split poin
benchmarks/haystacks/code/rust-library.rs