shootout-nbody improvements

- minimize bound check
- factorise operations
- use x, y, z instead of [f64, ..3]
- ~1.15 faster
auto merge of #14090 : TeXitoi/rust/shootout-nbody-improvement, r=alexcrichton

- minimize bound check
- factorise operations
- use x, y, z instead of [f64, ..3]
- ~1.15 faster