Future<T> type

Especially when when writing tests for async code, I often find myself needing a good ol' Future type, with an API equivalent to:

```swift
final class Future<T: Sendable>: Sendable
  func fulfill(_ value: T)
  var value: T { get async }
}
```

Instead of copy-pasting it between projects, I'd be happy to contribute a more polished version here. Would you be interested in it?