Introduce a way to construct Range from start + length

I understand this is an example and don’t want to get too deep into bikeshedding but do think it’s worth noting that Range::from_start_and_len is so absurdly verbose that I suspect nobody will ever use it, which somewhat defeats the point of something which is purely a convenience method.

If a ..+ b was implemented as syntax sugar for a .. (a + b) I think it would work exactly the way people would expect it to, and in those cases.