Reorganize and rewrite examples

This commit is a large-ish scale reorganization of our examples. The
main goal here is to have a dedicated section of the guide for example,
and all examples will be listed there. Each example's `README` is now
just boilerplate pointing at the guide along with a blurb about how to
run it.

Some examples like `math` and `smorgasboard` have been deleted as they
didn't really serve much purpose, and others like `closures` have been
rewritten with `web-sys` instead of hand-bound bindings.

Overall it's hoped that this puts us in a good and consistent state for
our examples, with all of them being described in the guide, excerpts
are in the guide, and they're all relatively idiomatically using
`web-sys`.

examples/dom/src/lib.rs

@@ -1,37 +1,25 @@
 extern crate wasm_bindgen;
+extern crate web_sys;
 
 use wasm_bindgen::prelude::*;
 
-// Definitions of the functionality available in JS, which wasm-bindgen will
-// generate shims for today (and eventually these should be near-0 cost!)
-//
-// These definitions need to be hand-written today but the current vision is
-// that we'll use WebIDL to generate this `extern` block into a crate which you
-// can link and import. There's a tracking issue for this at
-// https://github.com/rustwasm/wasm-bindgen/issues/42
-//
-// In the meantime these are written out by hand and correspond to the names and
-// signatures documented on MDN, for example
-#[wasm_bindgen]
-extern "C" {
-    type HTMLDocument;
-    static document: HTMLDocument;
-    #[wasm_bindgen(method)]
-    fn createElement(this: &HTMLDocument, tagName: &str) -> Element;
-    #[wasm_bindgen(method, getter)]
-    fn body(this: &HTMLDocument) -> Element;
-
-    type Element;
-    #[wasm_bindgen(method, setter = innerHTML)]
-    fn set_inner_html(this: &Element, html: &str);
-    #[wasm_bindgen(method, js_name = appendChild)]
-    fn append_child(this: &Element, other: Element);
-}
-
 // Called by our JS entry point to run the example
 #[wasm_bindgen]
-pub fn run() {
-    let val = document.createElement("p");
+pub fn run() -> Result<(), JsValue> {
+    // Use `web_sys`'s global `window` function to get a handle on the global
+    // window object.
+    let window = web_sys::window().expect("no global `window` exists");
+    let document = window.document().expect("should have a document on window");
+    let body = document.body().expect("document should have a body");
+
+    // Manufacture the element we're gonna append
+    let val = document.create_element("p")?;
     val.set_inner_html("Hello from Rust!");
-    document.body().append_child(val);
+
+    // 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())?;
+
+    Ok(())
 }