Reduce parallel processing boilerplate

[busstoptaktik]

Model parallel processing after this example from the Rayon docs

// https://docs.rs/rayon/1.1.0/rayon/slice/trait.ParallelSliceMut.html
use rayon::prelude::*;
let mut array = [1, 2, 3, 4, 5];
array.par_chunks_mut(2)
     .for_each(|slice| slice.reverse());
assert_eq!(array, [2, 1, 4, 3, 5]);

[busstoptaktik]

This should be implemented as part of a separate "parallel" Context