Upgrading spi to embedded_hal 1.0

[rubend056]

This is my follow up to #503, with your suggestions, since I needed the new Trait for a library I'm using. Perhaps we can continue to refine the implementation.

This might be good progress towards #468.

[Rahix]

Thanks a lot for picking up the changeset from the other PR. How much did you test this on real hardware so far?

Regarding the code, I only have one topic left to discuss, see below.

[rubend056]

I implemented the change by using SpiBus::flush(self)?; sice the Result is already infallible.

Now that I re-read the code, these function names are confusing raw_read and raw_read_buf, their name implies the only difference is one is a single byte read, and the other is multiple bytes read. But functionally one just reads spdr and the other makes a transaction on the bus by exchanging 0x00 with slave, reading incoming, repeating for every byte.

I understand this doesn't concern the end user of the library but it makes it confusing to understand by the maintainers.

I'll give this some rework and update accordingly.

[rubend056]

Ok, I've reworked the code, it should be functionally the same just slightly more clear.

I also moved the comments to the SpiOps definition so they would show up when hovering over the functions.

[rubend056]

The code was tested/re-tested with an Uno sending/receiving packets using the nrf24l01 module, register readings and transmission were successful. The functions directly used were transfer and transfer_in_place.

[Rahix]

Looking very good, thanks a lot!