embedded_io::serial::Reader implementations drop data on error

[jannic]

While working on #781 I noticed a flaw in the error handling of the reader implementation:

impl<D: UartDevice, P: ValidUartPinout<D>> embedded_io::Read for UartPeripheral<Enabled, D, P> {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize, Self::Error> {
        nb::block!(self.read_raw(buf)).map_err(|e| e.err_type)
    }
}

The error returned by read_raw does not only contain the error itself (in err_type) but also pub discarded: &'err [u8], possibly data that was already read from the peripheral before the error occurred. That data gets silently dropped.

Instead, it may be better to check if the length of discarded is non-zero, and if so, return an Ok result containing that data. In that case, the error needs to be stored somewhere, to be returned in the next call to Read.