Add visionOS support

[rofferom]

Just enable the same code blocks than on iOS.

[djc]

Hmm, I wonder if there is something better we can do, like treating all the Darwin stuff the same?

[rofferom]

Yes it would be great, especially because tvos support is missing too. I haven't tested it yet, that's why it isn't included in the MR.

[djc]

Do you know the right incantation for supporting everything Darwin at once?

[rofferom]

I have seen this one #[cfg(target_vendor = "apple")]. Other projects are using the same strategy than Quinn (ring, socket2).

Actually, I don't know the possible limitations of #[cfg(target_vendor = "apple")].

[djc]

#[cfg(target_vendor = "apple")] looks about right to me (I noticed std/core use it in a few places). @Ralith what do you think?

[rofferom]

I have just added tvOS support with the current way to declare it. Waiting for Ralith's feedback to update the MR if required.

[djc]

Curious to hear what you're building!

[Ralith]

#[cfg(target_vendor = "apple")] looks about right to me (I noticed std/core use it in a few places). @Ralith what do you think?

That seems very appealing. I don't know how we'd verify that it's exactly the semantics we'd want short of digging into rustc, but it's hard to imagine it's too far off. Maybe switch to it in a separate commit so we can easily revert if needed?