Rename and move macros in utils

[richardpringle]

From @iostat

Maybe I'm being an absurd level of pedantic here, but can these be called cfg_par(allel)_iter/_mut/_into_iter? Just the cfg_ alone makes it unclear which cfg is being triggered against...

Besides the name, love the macros. Would also consider moving them to their own src/macros.rs module

Originally posted by @iostat in #3522 (comment)

[richardpringle]

@iostat, what about maybe_par_iter? I'm usually all for parallel over par but par_iter makes me think of rayon right away, which is exactly what's happening.

And yes, I will move them

[richardpringle]

@iostat, ran out of time here. Assigning this to you. Should be a quick one