Protect finalizer references during execution

When we run finalizers we have to copy all of the finalizers to a new
data structure because a finalizer could add another finalizer and we
need to keep draining the "real" finalizer table until it's empty.
We don't want Ruby programs to mutate the finalizers that we're
iterating over as well.

Before this commit we would copy the finalizers in to a linked list.
The problem with this approach is that if compaction happens, the linked
list will need to be updated.  But the GC doesn't know about the
existence of the linked list, so it could not update references.  This
commit changes the linked list to be a Ruby array so that when
compaction happens, the arrays will automatically be updated and all
references remain valid.

gc.c

@@ -3370,12 +3370,7 @@ struct force_finalize_list {
 static int
 force_chain_object(st_data_t key, st_data_t val, st_data_t arg)
 {
-    struct force_finalize_list **prev = (struct force_finalize_list **)arg;
-    struct force_finalize_list *curr = ALLOC(struct force_finalize_list);
-    curr->obj = key;
-    curr->table = val;
-    curr->next = *prev;
-    *prev = curr;
+    rb_ary_push((VALUE)arg, rb_ary_new_from_args(2, (VALUE)key, (VALUE)val));
     return ST_CONTINUE;
 }
 
@@ -3402,16 +3397,18 @@ rb_objspace_call_finalizer(rb_objspace_t *objspace)
 
     /* force to run finalizer */
     while (finalizer_table->num_entries) {
-	struct force_finalize_list *list = 0;
-	st_foreach(finalizer_table, force_chain_object, (st_data_t)&list);
-	while (list) {
-	    struct force_finalize_list *curr = list;
-	    st_data_t obj = (st_data_t)curr->obj;
-	    run_finalizer(objspace, curr->obj, curr->table);
+        long i;
+	VALUE list = rb_ary_new_capa(finalizer_table->num_entries);
+
+	st_foreach(finalizer_table, force_chain_object, (st_data_t)list);
+
+        for (i = 0; i < RARRAY_LEN(list); i++) {
+            VALUE tuple = RARRAY_AREF(list, i);
+            VALUE obj = RARRAY_AREF(tuple, 0);
+            VALUE table = RARRAY_AREF(tuple, 1);
+	    run_finalizer(objspace, obj, table);
 	    st_delete(finalizer_table, &obj, 0);
-	    list = curr->next;
-	    xfree(curr);
-	}
+        }
     }
 
     /* prohibit GC because force T_DATA finalizers can break an object graph consistency */