I was looking at the rope implementation and some of the existing bugs
that crash in there, and I ran cargo-mutants to inspect test coverage. I
was motivated by bugs like
https://github.com/zed-industries/zed/issues/38556 but this doesn't fix
it and the bug may well be at a higher layer.

This PR adds coverage for a few functions that aren't tested today. I
didn't find any actual bugs yet.

I can see this tree is pretty sparse on docstrings so if you think these
are too verbose I can take them out or drop the whole PR.

Release Notes:

- N/A