ported AKS test for primes solution on rosettacode to Rust latest

I had 5 mins after lunch and I did a test of Rust archaeology (as per https://github.com/Hoverbear/rust-rosetta/issues/170). For these "algorithmic" sort of tasks the modernization looks easy enough, but maybe I got a particularly easy one...

It would be better to use `get` instead `get_mut` here.