feat: impl Default for row id types

src/_macros.rs

@@ -173,6 +173,12 @@ macro_rules! impl_id_traits {
                 self.partial_cmp(&other.0)
             }
         }
+
+        impl Default for $idtype {
+            fn default() -> Self {
+                Self::NULL
+            }
+        }
     };
 }
 

src/newtypes.rs

@@ -17,6 +17,9 @@ use bindings::tsk_size_t;
 /// use tskit::NodeId;
 /// use tskit::bindings::tsk_id_t;
 ///
+/// // The default value is null:
+/// assert_eq!(tskit::NodeId::default(), tskit::NodeId::NULL);
+///
 /// let x: tsk_id_t = 1;
 /// let y: NodeId = NodeId::from(x);
 /// assert_eq!(x, y);