We somehow ended up with a set hidden in `Task` as well as a set at top level. SILGen currently
hooks into the top-level ones, so shed the `Task`-namespaced versions for now.
Switch the contract between the runtime operation `swift_future_task_wait`
and Task.Handle.get() pver to an asynchronous call, so that the
compiler will set up the resumption frame for us. This allows us to
correctly wait on futures.
Update our "basic" future test to perform both normal returns and
throwing returns from a future, either having to wait on the queue or
coming by afterward.
Rather than immediately running the task synchronously within
runDetached, return the handle to the newly-created task. Add a method
task.Handle.run() to execute the task. This is just a temporary hack
that should not persist in the API, but it lets us launch tasks on a
particular Dispatch queue:
```swift
extension DispatchQueue {
func async<R>(execute: @escaping () async -> R) -> Task.Handle<R> {
let handle = Task.runDetached(operation: execute)
// Run the task
_ = { self.async { handle.run() } }()
return handle
}
}
```
One can pass asynchronous work to DispatchQueue.async, which will
schedule that work on the dispatch queue and return a handle. Another
asynchronous task can then read the result.
Yay for rdar://71125519.
Implement a new builtin, `cancelAsyncTask()`, to cancel the given
asynchronous task. This lowers down to a call into the runtime
operation `swift_task_cancel()`.
Use this builtin to implement Task.Handle.cancel().
