remove shootout warnings

Only one warning remain, and I can't find a way to remove it without doing more bound checks:

```
shootout-nbody.rs:105:36: 105:51 warning: use of deprecated item: use iter_mut, #[warn(deprecated)] on by default
shootout-nbody.rs:105             let bi = match b_slice.mut_shift_ref() {
```

using `split_at_mut` may be an option, but it will do more bound checking.

If anyone have an idea, I'll update this PR.