feat(ops): op2 support for Result<impl Future<...>>

[mmastrac]

Supports Result<impl Future<...>>-style async ops like so:

  #[op2(async, core)]
  pub fn op_async_result_impl(
    mode: u8,
  ) -> Result<impl Future<Output = Result<(), Error>>, Error> {
    if mode == 0 {
      return Err(generic_error("early exit"));
    }
    return Ok(async move {
      if mode == 1 {
        return Err(generic_error("early async exit"));
      }
      tokio::time::sleep(Duration::from_millis(500)).await;
      if mode == 2 {
        return Err(generic_error("late async exit"));
      }
      return Ok(());
    });
  }

[bartlomieju]

LGTM, provided the tests pass once lint error is fixed