Create separate work packet types for object tracing from mod buf

[qinsoon]

Currently all the tracing in a nursery GC is done in GenNurseryProcessEdges, including objects reachable from root, and objects reachable from mod buf. This PR uses seperate work packet types for objects that are from mod buf. This helps us understand the nursery GC time better.

[wks]

Alternatively, we can add a field in struct GenNurseryProcessEdges to identify their sources. For example,

pub struct GenNurseryProcessEdges<VM: VMBinding, P: GenerationalPlanExt<VM> + PlanTraceObject<VM>> {
    plan: &'static P,
    base: ProcessEdgesBase<VM>,
    source: PacketSource,
}

enum PacketSource {
    Root,
    ModBuf,
    RegionModBuf,
}