For `Void` element AsyncSequence types it might be nice to have a `waitForAll() async rethrows` similar to TaskGroup

```
extension AsyncSequence where Element == Void {
func waitForAll() async rethrows {
for try await _ in self { }
}
}
```

The proposed entry point is an AsyncSequence extension constrained to Element == Void; start by locating existing AsyncSequence extensions and related tests in the project. Add the shown waitForAll() behavior, then run the relevant tests or test suite to verify that it waits for all elements and propagates errors.

Written by the indexing model from the issue text.