The cancel token is severely lacking in basic functionality

[dead-claudia]

Things it needs:

1. Synchronous inspection. (If a cancel token has already been invoked, it shouldn't even attempt to run a worker method.)
2. Synchronous cancellation.
3. "Throw if cancelled", "resolve on cancellation", and "reject on cancellation".
4. Wrapping a generator to work like an async function, but bail on cancellation (while also notifying the generator).

[dead-claudia]

It might be better if I just roll my own dedicated CancelToken class to hold all the methods, and make this have something like this instead (basic implementation sketch):

// This holds all the real state
class CancelController {
  constructor(init) { ... }
  get isCanceled() { ... }
  get token() { ... } // memoized
  cancel() { ... }
  cancelAsync() { ... }
}

// Proxy for the controller, to provide privacy. Not directly exposed.
class CancelToken {
  constructor(ctrl) { ... }
  get isCanceled() { ... }
  get promise() { ... } // memoized
  throwIfCanceled() { ... }
  addCancel(func) { ... }
  removeCancel(func) { ... }
}

For compatibility, aborted and onabort should also be accepted in the API, in case someone uses a weird Abort{Signal,Controller} polyfill for some reason (and in case this gets ported to the browser, where that could legitimately happen).