This commit addresses #616 and replaces three occurrences of impl Iterator with a specific type. I do not see a benefit in the impl Iterator design pattern in our code base. Using a specific, public type is simpler for end-users, as discussed in #616.

impl Something only makes sense, I think, in use-cases where users specify code, such as in actix-web:

async fn greet(req: HttpRequest) -> impl Responder {
 let name = req.match_info().get("name").unwrap_or("World");
 format!("Hello {}!", &name)
}

Or in situations where the return type is complex and nested with many generics.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)