Add Promise(fulfilled:after:) and Promise(rejected:after:)

[lilyball]

We should have constructors Promise(fulfilled:after:), Promise(rejected:after:), and Promise(result:after:) that resolve after the given delay. This is distinct from e.g. Promise(fulfilled: foo).delay(3) in the way cancels are handled. With delay, if the upstream promise doesn't cancel (which Promise(fulfilled:) never will) then the delayed promise won't either. But with Promise(fulfilled:after:), requesting cancellation prior to the promise resolving will immediately cancel it.

[lilyball]

@julasamer This is now implemented on master, so you can start using it in place of Promise(fulfilled: ()).delay(1).