experiment with stackful for easier async interop

[jprochazk]

stackful is a library which exposes primitives to construct stackful coroutines and make them interop easily with an external async runtime. We could either try to use it directly, or construct a similar abstraction of our own. The gist of it is that we want to be able to do this:

// `eval` returns a future which is awaited
vm.eval("sleep(1)").await;

// inside, the execution of the VM is wrapped in `stackful`:
stackful(|| {
  vm.enter_dispatch_loop();
});

// at some point, `sleep` is called:
async fn sleep(scope: Scope<'_>) {
  tokio::time::sleep(1).await;
}

// which would normally cause the VM to yield all the way back up to
// the initial `eval`, but instead, the execution of the native async
// function is wrapped in a `wait` from `stackful`:
impl NativeAsyncFunction {
  fn call(&self, args: Args) -> Result<Value> {
    wait(self(args))
  }
}