transpile: simplify dynamic argument parsing

[afetisov]

Instead of it.next().unwrap().parse().unwrap() chains use slice patterns and a single expect call.

[kkysen]

I left some small suggestions on error messages and naming, but the main issue here is that previously bubble-up errors are now panicked, i.e. .ok_or(...)? was changed to .expect(...). I added suggestions to change these back to .map_err(|_| ...)? to get the previous behavior back. In the simd.rs case, .expect could be okay as the array is created statically and has exactly 3 elements, but .map_err(|_| ...)? is still better to keep consistency with the old behavior, and avoid introducing panics when refactoring later. Once it's changed to .map_err(|_| ...)?, this should be good to go.