Optimize &Iterator::nth

[Stebalien]

There are actually three changes here:

1. Remove unnecessary by_ref calls in the Iterator trait. I needed to remove the by_ref call from the nth method to make it object safe (by_ref isn't object safe) but I ended up removing all of them because they were pointless.
2. ⚠️ Make nth object safe. This changes nth's signature but I'm nearly positive that this change can't break existing code (see the commit message).
3. Override nth on by_ref iterators to make it call though to nth on the underlying implementation (the type behind the reference). This way, if the underlying implementation provides an optimized nth, by_ref iterators can take advantage of it.

Part of #24214

[rust-highfive]

r? @brson

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[Stebalien]

r? @aturon

[Stebalien]

Feel free to completely ignore this PR till after the 1.0 dust settles.

[alexcrichton]

Hm, I agree that I'm not 100% certain that the removal of where Self: Sized is OK to do, but I would personally suspect that it's compatible enough to be ok to land.

[alexcrichton]

@Stebalien could you back out the removal of where Self: Sized for Iterator::nth? After some discussion we're thinking that we may want to pursue this at a later date (and perhaps automatically not even require where Self: Sized for object safety), but for now it seems odd to make a one-off method on Iterator object-safe while leaving behind all the others.

[Stebalien]

@alexcrichton unfortunately, not without specialization. I would need I: Sized which rust happily informs me I'm not allowed to do:

error: the requirement `I : marker::Sized` appears on the impl method but not on the corresponding trait method [E0276]
fn nth(&mut self, n: usize) -> Option<I::Item> where I: Sized { (**self).nth(n) }

[alexcrichton]

Yeah the impl of Iterator for &mut I will no longer be able to delegate the nth implementation, unfortunately.