WIP: unrender error

WIP: not ready for merge
@Rich-Harris when the top-level of the fragment is a set of sections, it doesn't clear correctly. I tried faking `shouldDestroy` but that didn't help, which leads me to believe it's something in the fragment.unrender that needs to happen. There's a test in this PR for the issue.

I fixed the same issue we had with #928 which was lurking, but this one is a bit different.