Adjust successor iterators. Because rust-lang/rust#148054 was a slight perf regression. The problem was seemingly because this iterator structure: ``` slice_iter.chain(Option_iter.chain(Option_iter)) ``` changed to this: ``` slice_iter.chain(Option_iter).chain(Option_iter) ``` The commit also tweaks the `slice_iter` part, changing `into_iter` to `iter` and using `[]` instead of `(&[])`, for conciseness and consistency. r? `@saethlin`