Introduce a way to construct Range from start + length

kornel:

This could be added as a method to RangeFrom without being blocked by a new syntax:

(start..).take(len)

(start..).take(len) already exists for the old range types, it just returns std::iter::Take<RangeFrom<T>> instead of Range<T>. I think adding it for the new range types is a good idea since they don't directly implement Iterator