Process edges for a nursery GC. This type is provided if a generational plan does not use
crate::scheduler::gc_work::SFTProcessEdges. If a plan uses SFTProcessEdges,
it does not need to use this type.
The modbuf contains a list of objects in mature space(s) that
may contain pointers to the nursery space.
This work packet scans the recorded objects and forwards pointers if necessary.
The array-copy modbuf contains a list of array slices in mature space(s) that
may contain pointers to the nursery space.
This work packet forwards and updates each entry in the recorded slices.