Consider adding Promise.cancellingImmediatelyUponRequest()

[lilyball]

We might want to add a function like

extension Promise {
    /// Returns a new `Promise` that cancels immediately when requested.
    ///
    /// The new promise adopts the value of the receiver, unless the new promise
    /// is requested to cancel first. When `requestCancel()` is invoked the new
    /// promise will cancel itself immediately in addition to propagating the
    /// cancel upwards. This means the resulting promise will support
    /// cancellation even if the upstream promise doesn't.
    ///
    /// - Note: This method means the resulting promise can resolve to the
    /// `.cancelled` state even if the upstream promise is fulfilled or
    /// rejected.
    public func cancellingImmediatelyUponRequest() -> Promise {
        return Promise(on: .immediate, { (resolver) in
            let upstream = self.inspect(on: .immediate, { (result) in
                resolver.resolver(with: result)
            })
            resolver.onCancelRequested(on: .immediate) { [upstream=upstream.cancellable] (resolver) in
                resolver.cancel()
                upstream.requestCancel()
            }
        })
    }
}

It's not clear yet if this function is useful enough to justify its inclusion in the core library.