replace map with par_iter

---

examples/rayon/par-iter/src/main.rs

use rayon::prelude::*;

fn main() {
    let numbers = (1..10).collect::<Vec<_>>();

    let linear = numbers.iter().map(|number| number * 2).collect::<Vec<_>>();
    let parallel = numbers
        .par_iter()
        .map(|number| number * 2)
        .collect::<Vec<_>>();

    println!("{numbers:?}");
    println!("{linear:?}");
    println!("{parallel:?}");
    assert_eq!(linear, parallel);

    let _parallel = numbers
        .par_iter()
        .map(|number| {
            println!("{:?}", std::thread::current().id());
            number * 2
        })
        .collect::<Vec<_>>();
}

[1, 2, 3, 4, 5, 6, 7, 8, 9]
[2, 4, 6, 8, 10, 12, 14, 16, 18]
[2, 4, 6, 8, 10, 12, 14, 16, 18]
ThreadId(8)
ThreadId(9)
ThreadId(2)
ThreadId(17)
ThreadId(6)
ThreadId(5)
ThreadId(7)
ThreadId(3)
ThreadId(15)