Callback trait of scanning functions that directly trace through object graph edges.
An ObjectTracerContext gives a GC worker temporary access to an ObjectTracer, allowing
the GC worker to trace objects. This trait is intended to abstract out the implementation
details of tracing objects, enqueuing objects, and creating work packets that expand the
transitive closure, allowing the VM binding to focus on VM-specific parts.
Root-scanning methods use this trait to create work packets for processing roots.
VM-specific methods for scanning roots/objects.
Callback trait of scanning functions that report slots.