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/fetch/README.md

@@ -0,0 +1,16 @@
+# The `fetch` API
+
+[View documentation for this example online][dox]
+
+[dox]: https://rustwasm.github.io/wasm-bindgen/examples/fetch.html
+
+You can build the example locally with:
+
+```
+$ ./build.sh
+```
+
+(or running the commands on Windows manually)
+
+and then visiting http://localhost:8080 in a browser should run the example!
+

examples/fetch/build.bat

@@ -1,2 +0,0 @@
-cargo +nightly build --target wasm32-unknown-unknown
-cargo +nightly run --manifest-path ../../crates/cli/Cargo.toml --bin wasm-bindgen -- ../../target/wasm32-unknown-unknown/debug/fetch.wasm --out-dir .