Auto merge of #86744 - ijackson:sink-default, r=dtolnay

Auto merge of #86744 - ijackson:sink-default, r=dtolnay

impl Default, Copy, Clone for std::io::Sink and Empty

The omission of `Sink: Default` is causing me a slight inconvenience in a test harness.  There seems little reason for this and `Empty` not to be `Clone` and `Copy` too.

I have made all three of these insta-stable, because:

AIUI `Copy` can only be derived, and I was not able to find any examples of how to unstably derive it.  I think it is probably not possible.

I hunted through the git history for precedent and found

> 79b8ad84c84481a43704213cd0948d2ba0ea63b4
> Implement `Copy` for `IoSlice`
> https://github.com/rust-lang/rust/pull/69403

which was also insta-stable.