refactor: make DagRunner interface single-threaded

docs/COMPARISONS.md

@@ -15,7 +15,7 @@ impl Value { async fn run(&self) -> i32 { self.0 } }
 #[task]
 impl Add { async fn run(a: &i32, b: &i32) -> i32 { a + b } }
 
-let dag = DagRunner::new();
+let mut dag = DagRunner::new();
 let x = dag.add_task(Value(2));
 let y = dag.add_task(Value(3));
 let sum = dag.add_task(Add).depends_on((x, y));