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/build.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+set -ex
+
+# Two critical steps are required here to get this working:
+#
+# * First, the Rust standard library needs to be compiled. The default version
+#   is not compatible with atomics so we need to compile a version, with xargo,
+#   that is compatible.
+#
+# * Next we need to compile everything with the `atomics` feature enabled,
+#   ensuring that LLVM will generate atomic instructions and such.
+RUSTFLAGS='-C target-feature=+atomics' \
+  rustup run nightly xargo build --target wasm32-unknown-unknown --release
+
+# Threading support is disabled by default in wasm-bindgen, so use an env var
+# here to turn it on for our bindings generation. Also note that webpack isn't
+# currently compatible with atomics, so we go with the --no-modules output.
+WASM_BINDGEN_THREADS=1 \
+  cargo +nightly run --manifest-path ../../crates/cli/Cargo.toml \
+    --bin wasm-bindgen -- \
+    ../../target/wasm32-unknown-unknown/release/raytrace_parallel.wasm --out-dir . \
+    --no-modules
+
+python3 -m http.server