Trace the owner field of bindings (mmtk#169)

The `owner` field points to the module that a binding belongs to. It was
not traced. So if the module is moved, the pointer becomes invalid. The
alternative is to pin or use non moving allocation for all the module
objects here:
https://github.com/mmtk/julia/blob/d98aa333178d372d96f3c99c712edf0018a50289/src/module.c#L15.

Author: qinsoon

mmtk/src/julia_scanning.rs

@@ -64,10 +64,13 @@ pub unsafe fn scan_julia_object<SV: SlotVisitor<JuliaVMSlot>>(obj: Address, clos
             let value = ::std::ptr::addr_of!((*b).value);
             let globalref = ::std::ptr::addr_of!((*b).globalref);
             let ty = ::std::ptr::addr_of!((*b).ty);
+            let owner = ::std::ptr::addr_of!((*b).owner);
 
             process_slot(closure, Address::from_usize(value as usize));
             process_slot(closure, Address::from_usize(globalref as usize));
             process_slot(closure, Address::from_usize(ty as usize));
+            process_slot(closure, Address::from_usize(owner as usize));
+
             // clearing tag bits
             Address::from_usize(as_tagged_value).store::<usize>(t_header.as_usize() & !3);
             return;
@@ -234,10 +237,12 @@ pub unsafe fn scan_julia_object<SV: SlotVisitor<JuliaVMSlot>>(obj: Address, clos
             let value = ::std::ptr::addr_of!((*b).value);
             let globalref = ::std::ptr::addr_of!((*b).globalref);
             let ty = ::std::ptr::addr_of!((*b).ty);
+            let owner = ::std::ptr::addr_of!((*b).owner);
 
             process_slot(closure, Address::from_usize(value as usize));
             process_slot(closure, Address::from_usize(globalref as usize));
             process_slot(closure, Address::from_usize(ty as usize));
+            process_slot(closure, Address::from_usize(owner as usize));
             begin = begin.shift::<Address>(2);
         }