Investigate parallelization for wasm with `web_worker`

Use [`web_worker`](https://github.com/amethyst/web_worker) to create a new `rayon::ThreadPool` to enable parallelization on `wasm`.

The `wasm` app will need a initialization for the global ThreadPool, so probably something like this:

```rust
fn set_global_pool_to_web_workers() {
let concurrency = match web_sys::window() {
Some(window) => window.navigator().hardware_concurrency() as usize,
None => {
console_log!("Failed to get hardware concurrency from window. This function is only available in the main browser thread.");
2
}
};
let worker_pool = pool::WorkerPool::new(concurrency);
rayon::ThreadPoolBuilder::new()
.num_threads(concurrency)
.spawn_handler(|thread| Ok(pool.run(|| thread.run()).unwrap()))
.build()?;
}
```

cc @kobigurk @howardwu, this is of interest for speeding up proving and setup ceremonies in the browser.