Investigate parallelization for wasm with web_worker

[Pratyush]

Use 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:

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.

[Pratyush]

This makes it even easier to use rayon in wasm: https://github.com/GoogleChromeLabs/wasm-bindgen-rayon/