`allSettled` is out of sync with `Promise.allSettled`

[pgengler]

(Apologies if this has been addressed somewhere; I searched but didn't find anything.)

The allSettled implementation in rsvp returns an object with a state key to indicate whether the corresponding promise was "fulfilled" or "rejected". However, the Stage 4 TC39 proposal for Promise.allSettled uses status as the key, so the two aren't compatible.