Add Promise.makeChild()

[Lily Ballard]

When using Promise.propagatingCancellation(on:cancelRequested:) the client should always hand a child back to its caller. Right now the best way of producing a child looks like .then(on: .immediate, { _ in }) which is ugly and not necessarily obvious. We can add a .makeChild() method which produces a child that participates in automatic cancellation propagation, and then document this convenience on Promise.propagatingCancellation(on:cancelRequested:).