Implement support for WebAssembly threads

... and add a parallel raytracing demo!

This commit adds enough support to `wasm-bindgen` to produce a workable
wasm binary *today* with the experimental WebAssembly threads support
implemented in Firefox Nightly. I've tried to comment what's going on in
the commits and such, but at a high level the changes made here are:

* A new transformation, living in a new `wasm-bindgen-threads-xform`
  crate, prepares a wasm module for parallel execution. This performs a
  number of mundane tasks which I hope to detail in a blog post later on.
* The `--no-modules` output is enhanced with more support for when
  shared memory is enabled, allowing passing in the module/memory to
  initialize the wasm instance on multiple threads (sharing both module
  and memory).
* The `wasm-bindgen` crate now offers the ability, in `--no-modules`
  mode, to get a handle on the `WebAssembly.Module` instance.
* The example itself requires Xargo to recompile the standard library
  with atomics and an experimental feature enabled. Afterwards it
  experimentally also enables threading support in wasm-bindgen.

I've also added hopefully enough CI support to compile this example in a
builder so we can upload it and poke around live online. I hope to
detail more about the technical details here in a blog post soon as
well!

guide/src/SUMMARY.md

@@ -26,6 +26,7 @@
   - [web-sys: WebAudio](./examples/web-audio.md)
   - [web-sys: WebGL](./examples/webgl.md)
   - [web-sys: A Simple Paint Program](./examples/paint.md)
+  - [Parallel Raytracing](./examples/raytrace.md)
 - [Reference](./reference/index.md)
   - [Passing Rust Closures to JS](./reference/passing-rust-closures-to-js.md)
   - [Receiving JS Closures in Rust](./reference/receiving-js-closures-in-rust.md)

guide/src/examples/raytrace.md

@@ -0,0 +1,26 @@
+# Parallel Raytracing
+
+[View full source code][code] or [view the compiled example online][online]
+
+[online]: https://rustwasm.github.io/wasm-bindgen/exbuild/raytrace-parallel/
+[code]: https://github.com/rustwasm/wasm-bindgen/tree/master/examples/raytrace-parallel
+
+**This is an unstable and experimental example** of using threads with
+WebAssembly and Rust, culminating in a parallel raytracer demo. Current
+requirements for viewing this demo are:
+
+* Firefox Nightly - other browsers haven't implemented the proposed WebAssembly
+  features yet.
+* `SharedArrayBuffer` is enabled in `about:config` in Firefox
+
+This demo may also break over time as Firefox updates, but we'll try to keep it
+working!
+
+Locally to build this demo you'll need `xargo` and the `rust-src` rustup
+component, and afterwards `./build.sh` like other examples should build the
+example.
+
+Again, to reiterate, this is all experimental and we're working through various
+issues as we're working on this. If you're curious to see how this works it's
+best to explore via the source code right now! More info will be available here
+once WebAssembly threads are closer to stabilization.