Major improvements to wasm-bindgen-futures (#1760)

This PR contains a few major improvements:

* Code duplication has been removed.

* Everything has been refactored so that the implementation is much easier to understand.

* `future_to_promise` is now implemented with `spawn_local` rather than the other way around (this means `spawn_local` is faster since it doesn't need to create an unneeded `Promise`).

* Both the single threaded and multi threaded executors have been rewritten from scratch:

   * They only create 1-2 allocations in Rust per Task, and all of the allocations happen when the Task is created.

   * The singlethreaded executor creates 1 Promise per tick, rather than 1 Promise per tick per Task.

   * Both executors do *not* create `Closure`s during polling, instead all needed `Closure`s are created ahead of time.

   * Both executors now have correct behavior with regard to spurious wakeups and waking up during the call to `poll`.

   * Both executors cache the `Waker` so it doesn't need to be recreated all the time.

I believe both executors are now optimal in terms of both Rust and JS performance.

crates/futures/tests/tests.rs

@@ -71,7 +71,7 @@ async fn spawn_local_runs() {
 #[wasm_bindgen_test]
 async fn spawn_local_err_no_exception() {
     let (tx, rx) = oneshot::channel::<u32>();
-    spawn_local(async { });
+    spawn_local(async {});
     spawn_local(async {
         tx.send(42).unwrap();
     });