pub fn right_edge(self) -> Handle<NodeRef<BorrowType, K, V, NodeType>, marker::Edge> {
        unsafe { Handle::new_edge(self.node, self.idx + 1) }
    }
Given an edge index where we want to insert into a node filled to capacity,
computes a sensible KV index of a split point and where to perform the insertion.
The goal of the split point is for its key and value to end up in a parent node;
the keys, values and edges
benchmarks/haystacks/code/rust-library.rs