Add `AsyncPromiseWithTimeout` primitive and `asyncThen:otherwise:` method

[botwhytho]

• It would be useful to be able specify a timeout for a promise so that we're not waiting forever on computation.
• Also, seems like there is blocking then:otherwise: but not the equivalent non-blocking asyncThen:otherwise:. Seems like consistency should be good here so that user doesn't need to test with isRejectedor similar logic in their code.