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/rsx_usage.rs

@@ -21,7 +21,7 @@
 //!
 //! ### Events
 //! - Handle events with the "onXYZ" syntax
-//! - Closures can capture their environment with the 'a lifetime
+//! - Closures can capture their environment with the 'static lifetime
 //!
 //!
 //! ### Components