Multiple value return and actors, promises

There's no real reason to not support multiple value return.

(on (all-of (<- foo) (<- bar))
    (lambda (foo bar)
      'TODO))

(define car-and-noise-vow (<- factory 'make-car))
(define car-vow (first-promise-val car-and-noise-vow))
(<- car-vow 'drive)

It's created multiple problems to not have this, and the engineering team agrees we should do it now while it's feasible.