remove shootout warnings

[TeXitoi]

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.

[alexcrichton]

Yeah it looks like the recommended alternative, iter_mut won't quite work here because you'd almost want to clone/reverse a mutable iterator (which isn't quite sound). It's great to get rid of the other warnings, so we may as well land that in the meantime!

If we do end up removing mut_shift_ref, then it may wish to be included locally in the test as a safe function with an unsafe implementation, which is probably just fine as well!