Function alloc::collections::btree::node::splitpoint

fn splitpoint(edge_idx: usize) -> (usize, LeftOrRight<usize>)

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 to the left of the split point become the left child; the keys, values and edges to the right of the split point become the right child.