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!

examples/raytrace-parallel/worker.js

@@ -0,0 +1,32 @@
+// synchronously, using the browser, import out shim JS scripts
+importScripts('raytrace_parallel.js');
+
+let booted = false;
+let lastPtr = null;
+
+// Wait for the main thread to send us the shared module/memory. Once we've got
+// it, initialize it all with the `wasm_bindgen` global we imported via
+// `importScripts`.
+//
+// After our first message all subsequent messages are an entry point to run,
+// so we just do that.
+self.onmessage = function(args) {
+  self.onmessage = event => run(event.data);
+  const [module, memory] = args.data;
+  wasm_bindgen(module, memory)
+    .then(() => {
+      booted = true;
+      if (lastPtr)
+        run(lastPtr);
+    })
+    .catch(e => setTimeout(() => { throw e; })); // propagate to main `onerror`
+};
+
+function run(ptr) {
+  if (!booted) {
+    lastPtr = ptr;
+    return;
+  }
+  lastPtr = null;
+  wasm_bindgen.child_entry_point(ptr);
+}