correct call_residue_vars/3 using new copy_term_3 (#1239)

src/types.rs

@@ -469,6 +469,11 @@ impl HeapCellValue {
         )
     }
 
+    #[inline]
+    pub fn is_attr_var(self) -> bool {
+        self.get_tag() == HeapCellValueTag::AttrVar
+    }
+
     #[inline]
     pub(crate) fn as_var(self) -> Option<Ref> {
         read_heap_cell!(self,