util: Primarily fixes a bug where SmallVec.into_iter just doesn't work.

into_iter used to use `inline_size` as the capacity insetad of the actual
capacity. This patch fixes that, adds some utility methods to `SmallVec`
to bring it closer in functionality to `Vec`, and removes the obsolete
`owns_managed` calls.
Done. But I'm not confident in the logic I used, especially since it could lead to iterators with weird non-power-of-two capacities. I don't _think_ it's a problem, but I want to be sure.