Support asynchronous tests (#600)

* Tweak the implementation of heap closures

This commit updates the implementation of the `Closure` type to internally store
an `Rc` and be suitable for dropping a `Closure` during the execution of the
closure. This is currently needed for promises but may be generally useful as
well!

* Support asynchronous tests

This commit adds support for executing tests asynchronously. This is modeled
by tests returning a `Future` instead of simply executing inline, and is
signified with `#[wasm_bindgen_test(async)]`.

Support for this is added through a new `wasm-bindgen-futures` crate which is a
binding between the `futures` crate and JS `Promise` objects.

Lots more details can be found in the details of the commit, but one of the end
results is that the `web-sys` tests are now entirely contained in the same test
suite and don't need `npm install` to be run to execute them!

* Review tweaks

* Add some bindings for `Function.call` to `js_sys`

Name them `call0`, `call1`, `call2`, ... for the number of arguments being
passed.

* Use oneshots channels with `JsFuture`

It did indeed clean up the implementation!

crates/web-sys/tests/wasm/event.js

@@ -0,0 +1,10 @@
+export function new_event() {
+  return new Promise(resolve => {
+    window.addEventListener("test-event", resolve);
+    window.dispatchEvent(new Event("test-event", {
+      bubbles: true,
+      cancelable: true,
+      composed: true,
+    }));
+  });
+}