feat: enable addassign for usestate

examples/readme.rs

@@ -9,11 +9,11 @@ fn main() {
 }
 
 fn app(cx: Scope) -> Element {
-    let count = use_state(&cx, || 0);
+    let mut count = use_state(&cx, || 0);
 
     cx.render(rsx! {
         h1 { "High-Five counter: {count}" }
-        button { onclick: move |_| count.set(count + 1), "Up high!" }
-        button { onclick: move |_| count.set(count - 1), "Down low!" }
+        button { onclick: move |_| count += 1, "Up high!" }
+        button { onclick: move |_| count -= 1, "Down low!" }
     })
 }