Add AsyncFn family of traits

tests/ui/async-await/async-fn/simple.rs

@@ -0,0 +1,16 @@
+// edition: 2021
+// check-pass
+
+#![feature(async_fn_traits)]
+
+use std::ops::AsyncFn;
+
+async fn foo() {}
+
+async fn call_asyncly(f: impl AsyncFn(i32) -> i32) -> i32 {
+    f.async_call((1i32,)).await
+}
+
+fn main() {
+    let fut = call_asyncly(|x| async move { x + 1 });
+}