Implement WasmBindgen executor and reorganize

futures/src/executor/wasm_bindgen.rs

@@ -0,0 +1,17 @@
+use crate::Executor;
+
+#[derive(Debug)]
+pub struct WasmBindgen;
+
+impl Executor for WasmBindgen {
+    fn new() -> Result<Self, futures::io::Error> {
+        Ok(Self)
+    }
+
+    fn spawn(
+        &self,
+        future: impl futures::Future<Output = ()> + Send + 'static,
+    ) {
+        wasm_bindgen_futures::spawn_local(future);
+    }
+}