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.
use size hints
fix the build
auto merge of #3829 : cgaebel/servo/fix-into_iter, r=pcwalton

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.