feat(neon): Add tokio async runtime support

[kjvalencik]

This PR adds support for exporting async fn and future return fn in #[neon::export].

#[neon::export]
async fn example1(a: f64, b: f64) -> f64 {
    a + b
}

#[neon::export(async)]
fn example2(a: f64, b: f64) -> impl Future<Output = f64> {
    async move {
        a + b
    }
}

It is intentionally very conservative in semver guarantees. It requires registering a global future executor. The executor must implement the private Runtime trait. Currently, only a tokio implementation is included. Additionally, Future trait bounds are strict enough to allow for multi-threaded executors.

As we see demand, we can add support for additional executors (e.g., async-std) and possibly even single threaded executors (#[neon::export(?Sync)]) or a built-in libuv driven executor. The eventual goal is to make the Runtime trait public so that users can bring their own executor.

• Depends on improve(neon): Use autoref specialization for JSON wrapping return values in exported functions #1057 (Merged)

[dherman]

A few small changes, but nothing major. TBH I'm a little worried my lack of expertise at tokio and async means my review might be a little superficial. But the API is so easy to understand and fits so nicely with the #[export] macro syntax. This is a really exciting addition.

[dherman]

🚀