SpiBusDriver is not Send

[leonardodanelutti]

Is there a reason why Send is implemented for SpiDeviceDriver and not for a SpiBusDriver? From the documentation there seems to be no difference between a SpiBusDriver without cs and a SpiBusDriver.

[ivmarkov]

This should be fixed. PRs welcome!

Background: All drivers should be Send but currently few (if any) are Sync. We can actually also do the latter too for most drivers because it turns out most esp idf APIs are thread safe.

But usually folks reaching out to Sync often means they might be doing something suboptimal, as in - say - trying to use the default tokio work-stealing executor, which is not ideal on embedded.