Starts the fiber coroutine and waits until it is suspended or finishes.
message to be returned by the next suspend() call.
When the fiber is suspended, the message passed to that suspend() call. It has always an active member, by default num but never exc.
Exception if the fiber is suspended by suspendThrow().
In: The fiber must not be running (but waiting or finished).
See Implementation
Starts the fiber coroutine and waits until it is suspended or finishes.