async-await: add current_thread::Runtime::block_on_async (#1072)

This function is used by the Tokio macros introduced by #1058  but was
omitted from the PR.

async-await/tests/macros.rs

@@ -0,0 +1,22 @@
+#![feature(await_macro, async_await)]
+
+use tokio::await;
+use tokio::timer::Delay;
+use std::time::{Duration, Instant};
+
+#[tokio::test]
+async fn success_no_async() {
+    assert!(true);
+}
+
+#[tokio::test]
+#[should_panic]
+async fn fail_no_async() {
+    assert!(false);
+}
+
+#[tokio::test]
+async fn use_timer() {
+    let when = Instant::now() + Duration::from_millis(10);
+    await!(Delay::new(when));
+}