Consider optimizing map update for the shape-preserving case

One of the major costs of `update` is rebalancing on the way up. Notably, this has to follow pointers to the _siblings_ of all nodes along the path. We can avoid this in cases where the update neither inserts nor deletes a key by returning a flag saying so. This will obviously hurt a bit in cases where the shape _does_ change, but I conjecture that it won't hurt much.