Add <[T; N]>::copied

[scottmcm]

This works (it's a nicer version of how I did things in ArrayTools), so I figured I'd just send a PR and see what people think about it.

r? @LukasKalbertodt
cc #76118

[LukasKalbertodt]

I really want that method (and cloned), but I would prefer not using this kind of workaround :/
I think the correct solution would be like Option<T> does it: impl<T: Copy> Option<&T> { ... }. For that we need a way to add multiple impl blocks, which is not currently possible with only one lang item. I started a discussion on Zulip about this. Hopefully T-lang can help out :)

[matklad]

Out of curiosity, do we get "guaranteed" bounds check elimination here? That is, am I correct that rewriting this as dst.iter_mut().zip(self.iter()) would make this strictly worse?

[scottmcm]

After mir-opt the check is against a constant, which is quite easy for LLVM to remove. (The for i in 0..N loop is easily canonicalized, especially now that Range<usize>::next uses add nuw.)

[pickfire]

Must we write byte by byte if it implements copy? Oh, I forgot it is a reference not original value.

[pickfire]

I believe the next thing we will be adding is cloned?

[scottmcm]

Given #76236 (comment) I'll just close this.