Closes: #782

It would be preferable to avoid the code duplication between UartPeripheral and Reader. However, before starting a bigger code restructuring, I'd like to get some feedback on the general approach:

• Is it the right trade-off to reserve some memory in UartPeripheral and Reader to store the read error?
• What are alternative approaches?
  • Ignore the issue, accept discarding bytes on error. (Might be acceptable, assuming that serial errors are rare and the received message is probably garbled anyway. Should be clearly documented.)
  • Don't implement embedded_io::Read at all. (Breaking change, lost functionality, IMHO not acceptable)
  • Don't implement embedded_io::Read directly on UartPeripheral and Reader, but instead on a separate wrapper type. (Breaking change, slightly less nice API, zero-cost when not using embedded_io::Read)