Requested in: rolldown/rolldown#2016 (review)

I guess what's required is a parallel version of ivec.iter_enumerated() and ivec.into_iter_enumerated(), something like:

ivec.iter_enumerated().into_par_iter()
ivec.into_iter_enumerated().into_par_iter()

or

ivec.par_iter_enumerated()
ivec.into_par_iter_enumerated()

I'm not that great with rayon so I'm not sure what is best way. I'm surpised that ivec.iter_enumerated().into_par_iter() doesn't work "out of the box" already.