staticcheck: recommend iterator-based alternatives from the standard library

[dominikh]

If the various iterator-related Go proposals (such as golang/go#61901) get accepted, then code like for a, b := range strings.Fields(...) would be more efficient if it used iterator-based alternatives instead, because they wouldn't have to allocate slices.

This new check would very likely have to be opt-in. We don't want to start flagging 10 years of existing and previously correct code by default.