Implement Deref for all imported JS types

This commit implements the first half of [RFC #5] where the `Deref`
trait is implemented for all imported types. The target of `Deref` is
either the first entry of the list of `extends` attribute or `JsValue`.

All examples using `.as_ref()` with various `web-sys` types have been
updated to the more ergonomic deref casts now. Additionally the
`web-sys` generation of the `extends` array has been fixed slightly to
explicitly list implementatoins in the hierarchy order to ensure the
correct target for `Deref` is chosen.

[RFC #5]: https://github.com/rustwasm/rfcs/blob/master/text/005-structural-and-deref.md

examples/dom/src/lib.rs

@@ -16,10 +16,7 @@ pub fn run() -> Result<(), JsValue> {
     let val = document.create_element("p")?;
     val.set_inner_html("Hello from Rust!");
 
-    // Right now the class inheritance hierarchy of the DOM isn't super
-    // ergonomic, so we manually cast `val: Element` to `&Node` to call the
-    // `append_child` method.
-    AsRef::<web_sys::Node>::as_ref(&body).append_child(val.as_ref())?;
+    body.append_child(&val)?;
 
     Ok(())
 }