Compaction Hidden Set RAII Drop Guard

[marvin-j97]

Something like:

let _hidden = HiddenGuard::new(&levels, &to_compact);

HiddenGuard::drop() {
  let lock = self.levels.lock();
  lock.show_segments(self.ids);
}

That way, the drop guard shows hidden segments when an error is thrown with ?, currently it's error prone.

Considerations

Need to make sure, it cannot deadlock.

[marvin-j97]

This does not easily work because the hidden guard would write lock the already-locked level manifest on drop.
This needs to be reworked differently somehow.