Inverse square root

[tkaitchuck]

There are hardware instructions on X86 for approximate inverse square root and square root which can improved upon via newton's method.
See the for loop at the bottom of bodies_advance here: https://benchmarksgame-team.pages.debian.net/benchmarksgame/program/nbody-rust-7.html
It would look a lot better if this example could be re-written to use purely safe, normal looking rust code.