Merge #443

443: spi: make SpiDevice transaction take an operation slice instead of a closure. r=eldruin a=Dirbaio

As discussed in [today's WG meeting](https://matrix.to/#/!BHcierreUuwCMxVqOf:matrix.org/$l3MzRbzKQ0LGOops0ioYfM2kEr6-eT7ODboDj9iiCfY?via=matrix.org&via=psion.agg.io&via=tchncs.de).

- Fixes rust-embedded/linux-embedded-hal#87
- Gets rid of the cursed borrow issues in async.
- Makes the trait more consistent with I2C.


Co-authored-by: Dario Nieuwenhuis <[email protected]>

Author: bors[bot]