Accept an `AsyncSequence`

[Sindre Sorhus]

Just like we do with Progress.

When macOS 12 is out.

https://github.com/apple/swift-evolution/blob/main/proposals/0314-async-stream.md

I'm not yet sure what to call the API. I think we can just wait and see what kind of naming Apple decides to use for properties.

Actually, this is problematic as an AsyncSequence can only be consumed once. We'll have to wait for multi-consuming async sequences.