build(examples): add simple example

examples/simple.rs

@@ -0,0 +1,9 @@
+use mini_avl::Set;
+
+fn main() {
+    let mut set = Set::<i32>::new();
+    for i in 0..10 {
+        set.insert(i);
+    }
+    println!("{:#?}", set);
+}