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.rs

@@ -0,0 +1,29 @@
+use futures::future::Future;
+use js_sys::Promise;
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_futures::JsFuture;
+use wasm_bindgen_test::*;
+use web_sys::Event;
+
+#[wasm_bindgen(module = "./tests/wasm/event.js")]
+extern {
+    fn new_event() -> Promise;
+}
+
+#[wasm_bindgen_test(async)]
+fn event() -> impl Future<Item = (), Error = JsValue> {
+    JsFuture::from(new_event())
+        .map(Event::from)
+        .map(|event| {
+            // These should match `new Event`.
+            assert!(event.bubbles());
+            assert!(event.cancelable());
+            assert!(event.composed());
+
+            // The default behavior not initially prevented, but after
+            // we call `prevent_default` it better be.
+            assert!(!event.default_prevented());
+            event.prevent_default();
+            assert!(event.default_prevented());
+        })
+}