Return of <<- (eg, "await" / coroutines)

Here I'm considering returning adding coroutines to Goblins. They'd work a bit differently than before, allowing composition in environments when called with $ but without being able to interfere with anyone lower on the stack, thereby blocking re-entrancy attacks.

Instead of suspending to the top of the turn, an actor using <<- would suspend to the top of its own call and would return a promise.

In effect, the following code would be equivalent:

(define ((^alice bcom bob))
  (displayln "before")
  (define from-bob (<<- bob "hello"))
  (displayln "after"))

(define ((^alice bcom bob))
  (displayln "before")
  (on (<- bob "hello")
      (lambda (from-bob)
        (displayln "after"))
      #:promise? #t))

<<- should be interpreted not only as await but also as potentially "oh yeah I'm going so fast oh shiiiii" for the usual reasons: it's different than $ in that the world is likely to change on you.

The choice to return #:promise? #t probably will DTRT by default with asynchronous stuff due to promise pipelining/chaining/etc, and it also provides a path out for doing something sensible with $.