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/test/src/rt/detect.rs

@@ -1,7 +1,7 @@
 //! Runtime detection of whether we're in node.js or a browser.
 
 use wasm_bindgen::prelude::*;
-use js_sys::{Array, Function};
+use js_sys::Function;
 
 #[wasm_bindgen]
 extern {
@@ -54,7 +54,7 @@ pub fn is_browser() -> bool {
     //
     // Whew!
     let this = Function::new_no_args("return this")
-        .apply(&JsValue::undefined(), &Array::new())
+        .call0(&JsValue::undefined())
         .unwrap();
     assert!(this != JsValue::undefined());
     This::from(this).self_() != JsValue::undefined()