Remove ptr/ref to Plan from work packets

[wks]

GCWork::do_work has an mmtk parameter, and the MMTK::get_plan() and MMTK::get_plan_mut() methods provide access to the plan instance. It is therefore pointless for a work packet to capture a pointer or reference inside the struct itself.

This PR does not fix the fact that Plan::prepare and Plan::release have &mut self arguments. Therefore, getting &mut Plan from an MMTK instance is still unsafe. See:
#852

[wks]

I am not sure if it is worth making this change. It is always safer to use fewer raw pointers and weird casting statements like unsafe { &mut *(self.plan as *const _ as *mut _) }. But this PR doesn't really make mmtk.get_plan() or mmtk.get_plan_mut() safe because MMTK::plan is still an UnsafeCell<Box<dyn Plan<VM = VM>>>.

[qinsoon]

I am not sure if it is worth making this change. It is always safer to use fewer raw pointers and weird casting statements like unsafe { &mut *(self.plan as *const _ as *mut _) }. But this PR doesn't really make mmtk.get_plan() or mmtk.get_plan_mut() safe because MMTK::plan is still an UnsafeCell<Box<dyn Plan<VM = VM>>>.

I think it is cleaner. With the refactoring, all we need to address is the unsafe on MMTK::get_plan/get_plan_mut.