Introduce a way to construct Range from start + length

I know that in general using inclusive ranges are slower than exclusive ranges, since the type needs to keep track of if it reached the end separately (in case the ending value is one that would overflow).

However, I don't know if rustc can optimise this when the range is known at compile time to not overflow. Otherwise you really should be writing 'a'..'{', which is not ideal for human readers.