stkbuf_slot and exec obj are always transitively pinned

[qinsoon]

See

mmtk-julia/mmtk/src/julia_scanning.rs

Line 426 in 6e3eb37

process_slot(closure, stkbuf_slot);

mmtk-julia/mmtk/src/julia_scanning.rs

Line 449 in 6e3eb37

Address::from_mut_ptr(closure),

closure is the TPIN closure from

mmtk-julia/mmtk/src/scanning.rs

Line 67 in 6e3eb37

&mut tpinning_slot_buffer,

I am not sure if we have to use TPIN for those two cases, and I don't know how many objects will be transitively pinned from those two places.