Rust SIMD on stable

[darleybarreto]

This change allows to run the Rust SIMD code on stable Rust.

[darleybarreto]

I was trying to make a version of this to run in parallel. But I couldn't figure out a easy way without using message passing, Mutex or Arc. I wish I know more Rust to have a easy way out. Wrt this PR, idk how good it will perform, since idk how wide works in general with the several SIMD features out there.

[darleybarreto]

@paugier what you think? Could this be merged as is?

[darleybarreto]

@paugier I just added a parallel version for Rust. On my machine I got the following

time ./nbabel-nbody/target/release/nbabel ../data/input2k 2
Running Parallel SIMD version
Final dE/E = -0.44307369910184735

real    0m40,392s
user    4m41,494s
sys     0m1,242s

time julia -t8 -O3 --check-bounds=no -- run.jl nbabel5_threads.jl ../data/input2k false

real    0m34,271s
user    4m2,255s
sys     0m1,009s

Since it's a very naive and rough translation of the nbabel5_threads.jl, I'd expect there's room to improve there. For the 1k file, both Rust and Julia have similar performance!