ThinBox::clone() leaks Head if the iterator panics

[adlerd]

A typo in ThinBox::clone() causes it to drop_in_place a u8, not a Head.

A test which demonstrates this is here.

I will follow up with a PR adding this test and fixing the issue, which are probably the best way to explain it, although I didn't give my full attention to make sure the fix is doing the right thing so someone should look closely :). I think a more complete fix would be to remove the Head from InProgressThinBox entirely as I'm not sure it really needs to be there.

[CAD97]

FWIW, this prompted me to go clean up my panics over in slice-dst as well.