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

guide/src/web-sys/inheritance.md

@@ -0,0 +1,55 @@
+# Inheritance in `web-sys`
+
+Inheritance between JS classes is the bread and butter of how the DOM works on
+the web, and as a result it's quite important for `web-sys` to provide access to
+this inheritance hierarchy as well! There are few ways you can access the
+inheritance hierarchy when using `web-sys`.
+
+### Accessing parent classes using `Deref`
+
+Like smart pointers in Rust, all types in `web_sys` implement `Deref` to their
+parent JS class. This means, for example, if you have a `web_sys::Element` you
+can create a `web_sys::Node` from that implicitly:
+
+```rust
+let element: &Element = ...;
+
+element.append_child(..); // call a method on `Node`
+
+method_expecting_a_node(&element); // coerce to `&Node` implicitly
+
+let node: &Node = &element; // explicitly coerce to `&Node`
+```
+
+Using `Deref` allows ergonomic transitioning up the inheritance hierarchy to the
+parent class and beyond, giving you access to all the methods using the `.`
+operator.
+
+### Accessing parent classes using `AsRef`
+
+In addition to `Deref`, the `AsRef` trait is implemented for all types in
+`web_sys` for all types in the inheritance hierarchy. For example for the
+`HtmlAnchorElement` type you'll find:
+
+```rust
+impl AsRef<HtmlElement> for HtmlAnchorElement
+impl AsRef<Element> for HtmlAnchorElement
+impl AsRef<Node> for HtmlAnchorElement
+impl AsRef<EventTarget> for HtmlAnchorElement
+impl AsRef<Object> for HtmlAnchorElement
+impl AsRef<JsValue> for HtmlAnchorElement
+```
+
+You can use `.as_ref()` to explicitly get a reference to any parent class from
+from a type in `web_sys`. Note that because of the number of `AsRef`
+implementations you'll likely need to have type inference guidance as well.
+
+### Accessing child clases using `JsCast`
+
+Finally the `wasm_bindgen::JsCast` trait can be used to implement all manner of
+casts between types. It supports static unchecked casts between types as well as
+dynamic runtime-checked casts (using `instanceof`) between types.
+
+More documentation about this can be found [on the trait itself][jscast]
+
+[jscast]: https://docs.rs/wasm-bindgen/0.2/wasm_bindgen/trait.JsCast.html