When built with tokio, provide a SingleThreadExecutor

An application can (optionally) use this to spawn a single tokio thread
instead of one per processor thread.

src/single_thread_executor.rs

@@ -0,0 +1,28 @@
+use std::{future::Future, thread};
+
+#[cfg(feature = "tokio")]
+pub struct SingleThreadExecutor(tokio::runtime::Runtime);
+
+#[cfg(feature = "tokio")]
+impl iced_native::Executor for SingleThreadExecutor {
+    fn new() -> Result<Self, iced::futures::io::Error> {
+        // Current thread executor requires calling `block_on` to actually run
+        // futures. Main thread is busy with things other than running futures,
+        // so spawn a single worker thread.
+        Ok(Self(
+            tokio::runtime::Builder::new_multi_thread()
+                .worker_threads(1)
+                .enable_all()
+                .build()?,
+        ))
+    }
+
+    fn spawn(&self, future: impl Future<Output = ()> + Send + 'static) {
+        let _ = self.0.spawn(future);
+    }
+
+    fn enter<R>(&self, f: impl FnOnce() -> R) -> R {
+        let _guard = self.0.enter();
+        f()
+    }
+}