cargo fmt rust code (#1131)

* cargo fmt code

* Add empty line to seperate unrelated comments

* Fix review

* Update bubble_sort.rs

* Update merge_sort.rs

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Co-authored-by: Yudong Jin <krahets@163.com>

codes/rust/chapter_tree/binary_tree_dfs.rs

@@ -68,4 +68,4 @@ fn main() {
     /* 后序遍历 */
     let vec = post_order(root.as_ref());
     print!("\n后序遍历的节点打印序列 = {:?}", vec);
-}
+}