macros: add pin! macro (#2163)

Used for stack pinning and based on `pin_mut!` from the pin-util crate.

Pinning is used often when working with stream operators and the select!
macro. Given the small size of `pin!` it makes more sense to include a
version than re-export one from a separate crate or require the user to
depend on `pin-util` themselves.

tests-integration/tests/macros_pin.rs

@@ -0,0 +1,12 @@
+use futures::executor::block_on;
+
+async fn my_async_fn() {}
+
+#[test]
+fn pin() {
+    block_on(async {
+        let future = my_async_fn();
+        tokio::pin!(future);
+        (&mut future).await
+    });
+}