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.

src/executor.rs

@@ -7,13 +7,23 @@ pub use platform::Default;
 mod platform {
     use iced_futures::{executor, futures};
 
-    #[cfg(feature = "tokio")]
+    #[cfg(feature = "tokio_old")]
+    type Executor = executor::TokioOld;
+
+    #[cfg(all(not(feature = "tokio_old"), feature = "tokio"))]
     type Executor = executor::Tokio;
 
-    #[cfg(all(not(feature = "tokio"), feature = "async-std"))]
+    #[cfg(all(
+        not(any(feature = "tokio_old", feature = "tokio")),
+        feature = "async-std"
+    ))]
     type Executor = executor::AsyncStd;
 
-    #[cfg(not(any(feature = "tokio", feature = "async-std")))]
+    #[cfg(not(any(
+        feature = "tokio_old",
+        feature = "tokio",
+        feature = "async-std"
+    )))]
     type Executor = executor::ThreadPool;
 
     /// A default cross-platform executor.

src/lib.rs

@@ -193,10 +193,17 @@ pub mod widget;
 pub mod window;
 
 #[cfg(all(
-    any(feature = "tokio", feature = "async-std"),
+    any(feature = "tokio", feature = "tokio_old", feature = "async-std"),
     not(target_arch = "wasm32")
 ))]
-#[cfg_attr(docsrs, doc(cfg(any(feature = "tokio", feature = "async-std"))))]
+#[cfg_attr(
+    docsrs,
+    doc(cfg(any(
+        feature = "tokio",
+        feature = "tokio_old",
+        feature = "async-std"
+    )))
+)]
 pub mod time;
 
 #[cfg(all(