Add support for cancellable Promise in await operator

[tcurdt]

Let's say I use the example:

let asyncFunc = async({ _ -> Int in
  let loggedUser = try await(loginUser(username,pass))
  let followersList = try await(getFollowers(loggedUser))
  let countUnfollowed = try await(unfollow(followersList))
  return countUnfollowed
}).then({ value in
  print("Unfollowed \(value) users")
})

How would I make that cancelable?

[malcommac]

Thank you for your request; I've added it in 0.9.91.
Below a short example used in unit test.

	func test_invalidationTokenWithAsyncOperator() {
		let exp = expectation(description: "test_retry_condition")
		let invalidator: InvalidationToken = InvalidationToken()
		
		async(token: invalidator, { st -> String in
			Thread.sleep(forTimeInterval: 2.0)
			if st.isCancelled {
				print("Promise cancelled")
				exp.fulfill()
			} else {
				print("Promise resolved")
				XCTFail()
			}
			return ""
		}).then { _ in

		}
		invalidator.invalidate()
		waitForExpectations(timeout: expTimeout, handler: nil)
	}

[tcurdt]

@malcommac thanks for looking into this. TBH I don't fully grasp the usage and how to apply it to the above snipped though.