Add Rayon multithreading in `jordan_wigner.rs`

[davidomanovic]

Btw @davidomanovic if this can be multithreaded in Rust (for example using Rayon) that would be a welcome contribution too. I just didn't want to have Python-level multiprocessing for now.

Originally posted by @kevinsung in #484 (comment)

[kevinsung]

Closed in #497