[RFC] Provide wrenGetCurrentFiber() API

[joshgoebel]

This would be helpful for schedulers in general but more specifically would solve this problem of always needing Fiber.current inside C for callbacks - but never having it... resulting in code like:

Scheduler.await_ { startTimer_(milliseconds, Fiber.current) }

It would be desirable to be able to simplify:

Scheduler.await_ { startTimer_(milliseconds) }

This can be done now fairly easily by accessing private API but would be fragile in the future and subject to breakage:

WrenHandle* getFiberCurrent(WrenVM* vm) {
   return wrenMakeHandle(vm, OBJ_VAL(vm->fiber));
}

It's also possible for the Scheduler to just push this information down into C itself:

  static await_(fn) {
    preserveFiberCurrent_(Fiber.current)
    fn.call()
    return Scheduler.runNextScheduled_()
  }

Having the API already available in C to achieve this seems a much more elegant solution though. If this would be welcome I can create a PR.

---

Original context:

Another thing we can do, however, is to expose a wrenGetCurrentFiber() method in the API (because it's useful for schedulers in general).

Originally posted by @ChayimFriedman2 in #102 (comment)

[ChayimFriedman2]

I think this issue should target the core repository, since we're talking about adding a function there.

[joshgoebel]

That was my intent, and my mistake. Closing and will re-open there. :-)