Add Application::Executor associated type

futures/src/executor/null.rs

@@ -0,0 +1,13 @@
+use crate::Executor;
+
+use futures::Future;
+
+pub struct Null;
+
+impl Executor for Null {
+    fn new() -> Result<Self, futures::io::Error> {
+        Ok(Self)
+    }
+
+    fn spawn(&self, _future: impl Future<Output = ()> + Send + 'static) {}
+}