[Feature] Improve device detection and UART device process

[laurensvalk]

Is your feature request related to a problem? Please describe.
The device connection manager (DCM) and LEGO UART Devices (pup_uart) currently run as parallel processes that have to exchange some data to inform the other when to start and stop doing things.

It has been that way historically but there is probably room for improvement in future releases.

Describe the solution you'd like

Instead of two parallel processes, maybe we need just one process (which pushes N "processes" along). In pseudo-code, perhaps it could look like this for each sensor.

while True:
    await passive_device_detection_until_uart_detected()
    success = await pup_uart_do_sync_phase()
    if not success:
        continue
    await gather ( pup_uart_receive_data_until_no_data() , pup_uart_send_data_and_ack() , race = True)

• Explicit synchronization or messaging is not needed this way.
• the pup_uart code is entirely independent and can be used on ev3 too, etc.

[laurensvalk]

This is looking easier than expected, so I'll give this a try for review.

[laurensvalk]

This is started in https://github.com/pybricks/pybricks-micropython/commits/uart-process but needs some further cleanups.

Instead of synchronization signals, we now have:

static PT_THREAD(pbdrv_legodev_pup_thread(ext_dev_t * dev)) {

    PT_BEGIN(&dev->pt);

    while (true) {

        // Initially assume nothing is connected.
        dev->connected_type_id = PBDRV_LEGODEV_TYPE_ID_NONE;
        dev->dcm.dev_id_match_count = 0;

        // Run the device detection manager until a UART device is detected.
        etimer_set(&dev->timer, 2);
        PT_WAIT_UNTIL(&dev->pt, ({
            if (etimer_expired(&dev->timer)) {
                etimer_reset(&dev->timer);
                (void)PT_SCHEDULE(poll_dcm(dev));
                debug_state_change(dev);
                dev->prev_type_id = dev->connected_type_id;
            }
            dev->connected_type_id == PBDRV_LEGODEV_TYPE_ID_LPF2_UNKNOWN_UART;
        }));

        // UART device detected, so run that protocol until it disconnects.
        legodev_pup_enable_uart(dev->pins);
        PT_SPAWN(&dev->pt, &dev->uart_dev_pt, pbdrv_legodev_pup_uart_thread(&dev->uart_dev_pt, dev->uart_dev));
    }
    PT_END(&dev->pt);
}```

I have intentionally avoided changes to the `dcm` and `uartdev` code in this commit to make the change easier to process.

[laurensvalk]

This is completed and merged.