Remove env_dict param from Syscall.CallAsync callbacks

`Syscall.CallAsync` expects a callback with signature `callback({env_dict}, {result_dict})`, where env_dict contains cursor and vim state info. In practice, it's not easy to remember the argument order, and it's easy enough and more explicit/flexible for the caller to just capture the env info they need and bind it to the callback with `maktaba#function#Create`.

We should migrate `Syscall.CallAsync` to expect a signature `callback({result_dict})` instead and just document how callers can capture and pass info along.