Introduce tokio_old feature

This feature allows users to rely on the `0.2` version of `tokio` while
the async ecosystem upgrades to the latest version.

futures/src/executor/tokio_old.rs

@@ -0,0 +1,21 @@
+use crate::Executor;
+
+use futures::Future;
+
+/// An old `tokio` runtime.
+#[cfg_attr(docsrs, doc(cfg(feature = "tokio_old")))]
+pub type TokioOld = tokio_old::runtime::Runtime;
+
+impl Executor for TokioOld {
+    fn new() -> Result<Self, futures::io::Error> {
+        tokio_old::runtime::Runtime::new()
+    }
+
+    fn spawn(&self, future: impl Future<Output = ()> + Send + 'static) {
+        let _ = tokio_old::runtime::Runtime::spawn(self, future);
+    }
+
+    fn enter<R>(&self, f: impl FnOnce() -> R) -> R {
+        tokio_old::runtime::Runtime::enter(self, f)
+    }
+}