Introduce a way to construct Range from start + length

Also, @appleGun22, since you’re new here, note that in order to add Range::from_len(), you don't necessarily have to convince the audience here on internals.rust-lang.org; you only need to convince the standard library team, which takes API Change Proposals. In order to write a successful ACP you would need to flesh out your proposal a bit more, particularly with real-world examples of code it would improve, and the feedback here may help you prepare.