* Move SPI-related traits into their own preludes in the `spi` module
* Remove the `embedded-hal-async` trait re-exports and the `eh1` module from the prelude
* Update `CHANGELOG.md`
* Refactor the `spi` and `spi_slave` modules into a single `spi` module
* Update import paths in examples as needed
* Extract some common types from `spi::master` into the root `spi` module
* Fix imports in examples (again)
* Documentation fixes/improvements
* Remove unnecessary re-exports
* Update `CHANGELOG.md`