extra::tempfile::mkdtemp should return an RAII wrapper, not a Path

There is not a lot of ways to use the path to a supposedly temporary filesystem entry correctly without some RAII'd logic to remove the entry in the case of failure. Pretty much all callers have to use `finally`, which I gather we aren't too happy with because of rightwards drift, or roll their own RAII, which seems like it could be provided out of the box.
