Introduce a way to construct Range from start + length

appleGun22:

Doesn't have a possibility to express an invalid (start > end) range

It still does, just via things like 200_u8 ..+ 100.

appleGun22:

Is generally the more natural way to express a range

Unfortunately it works very poorly with general ranges in other places. "a".."b" works fine, and is even used in things like BTreeMap methods.

But if you had something like "a" ..+ 10, what does that mean? It clearly can't implement RangeBounds in std::ops - Rust, for example.

I think if you want the "counted" part, separating it out is useful. my_btree.range(start..).take(count), for example, works well.

appleGun22:

Reduce boilerplate (repeating start)

How about just a function? Range::from_start_and_len(start, length) would be fine, no new type.