Create closure type; allow async event handlers in props; allow short hand event handlers (#2437)

* create closure type; allow async event handlers in props; allow shorthand event handlers

* test forwarding event handlers with the shorthand syntax

* fix clippy

* fix imports in spawn async doctest

examples/calculator_mutable.rs

@@ -89,7 +89,7 @@ fn app() -> Element {
 #[component]
 fn CalculatorKey(name: String, onclick: EventHandler<MouseEvent>, children: Element) -> Element {
     rsx! {
-        button { class: "calculator-key {name}", onclick: move |e| onclick.call(e), {&children} }
+        button { class: "calculator-key {name}", onclick, {children} }
     }
 }