This defines a GC work packet which are assigned to the
GCWorkers by the scheduler.
Work packets carry payloads that indicate the work to be done. For example, a work packet may
contain a pointer to a stack that must be scanned, or it may contain a large buffer of pointers
that need to be traced, or it might contain a range of static variables to be scanned, etc. The size
of the work packet will need to consider at least two points of tension: the work packet must be large
enough to ensure that the costs of managing the work packets do not dominate, and the packet must be
small enough that good load balancing is achieved.