- Should we have a nanosecond-precision delay trait? yes/no
  - if yes, i'd make it be a single same trait. Perhaps rename DelayUs to SimpleDelay or BasicDelay, add `delay_ns`.
- Should SPI transaction DelayUs be DelayNs? yes/no
  - imo no, it shouldn't be used for CS delays. Even if we do have a trait with `delay_ns`.