Introduce a way to construct Range from start + length

There is a natural way to define steps on strings which is used frequently in theoretical computer science for algorithms/proofs which enumerate all strings: you simply change the ordering such that all shorter strings appear before all longer strings (equivalently, left pad the strings with “-1 characters” to the same length before comparison). Unfortunately, this would be mismatched with Rust’s Ord implementation for strings/vectors so cannot be used to implement Step for strings.

Either way, however, I’m hard pressed to imagine a case where what you want is a range of specific length over strings rather than characters since even under this definition, between "z" and "aa" you have an annoying to compute number of hundreds/thousands of strings including non-alphabetic and, more broadly, non-printable characters that you’d need to account for.