Make SerialOutstation::open synchronous

[jadamcrain]

Problem

The new unstable SerialOutstation::open API is declared async:

pub async fn open(
    self,
    path: &str,
    settings: SerialSettings,
) -> std::io::Result<SerialOutstationTask>

The implementation contains no await. It calls crate::serial::open, which calls the synchronous tokio_serial::SerialStream::open, and then constructs the task.

The async signature suggests that opening the port is performed asynchronously, and it requires callers to create and drive a future even though the operation completes synchronously.

Proposed change

Make open a synchronous method:

pub fn open(
    self,
    path: &str,
    settings: SerialSettings,
) -> std::io::Result<SerialOutstationTask>

The API is currently behind the unstable feature, so this is a good time to correct the signature before stabilization.