add spi listen callback and stop listen callback function - #1124

Thanks for this. I will review it soon.

Sorry for the long wait to review this...

The spi.listen method that you added is essentially the same as spi.recv except that it returns before the receive finishes (if it's a DMA transfer). And then a callback gets called when the receive is actually done.

I think this functionality should be part of spi.recv, enabled with a keyword something like block=False. This would scale to the send method, and also the I2C peripheral.

But the way to handle callbacks in general is still blocked pending discussion in #1118.

Given that this PR is so old I'm going to close it.

The idea is good -- to support asynchronous SPI transfers with callbacks -- but the overall machine API is not there yet. When it is hopefully a variant of this PR will be implemented.