Tracking issue for `step_by` stabilization

[aturon]

Update (@SimonSapin): this is now the tracking issue for an iterator adaptor based on Iterator::nth:

pub trait Iterator {
    fn step_by(self, step: usize) -> StepBy<Self>
}

The Step trait used for making ranges iterators is now tracked at #42168.

Original issue:

---

The step_by method makes it possible to step through ranges with arbitrary-sized steps. There are several issues that need to be addressed before we can stabilize it:

• The design/stabiliztion of the Step trait, which is currently a bit of a mess
• The behavior on negative steps (see this thread)
• Likely the design/stabilization of Zero/One, tracked here

[aturon]

Nominating for P-High.

[alexcrichton]

triage: P-high

[huonw]

cc #23588

[sfackler]

We should probably start moving on this in the next cycle or drop the P-high.

[aturon]

Would definitely like some help getting this over the finish line. I'm happy to mentor!