Don't split small nodes

[deepinthebuild]

We should change the behavior of insert etc. to not split leaves that are below a certain size threshold. It might help with performance due to cache friendliness.

[hawkw]

I'm down for this; but I'm not gonna make it a priority, just because I think it could take a lot of effort to implement under the hood.

[hawkw]

Having done some profiling, though, it does look like this is close to the only big performance optimisation we have left – Rope.split() does spend a majority of it's time in &str.to_owned(). Avoiding splits on small strings might help us here...