Registers the specified fulfillment and rejection handlers.
If the promise is already resolved, they are called
as soon as possible (but not immediately).
Promise!(Unpromise!R, F)then(R delegate(A) nothrow onFulfilled, R delegate(E) nothrow onRejected)
Registers the specified fulfillment and rejection handlers. If the promise is already resolved, they are called as soon as possible (but not immediately).