Move the original get_draw_order code into the tree_iter module.

src/nix_util/activity_tree.rs

@@ -6,7 +6,7 @@ use std::ops::Deref;
 use super::activity::Activity;
 use super::activity::ActivityRoot;
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq)]
 pub(crate) struct ActivityId {
     id: u64,
 }