implement embedded_io::ReadReady and WriteReady traits for uart peripheral

[antbern]

Implements embedded_io::ReadReady and WriteReady traits for split and unsplit uart peripheral in terms of the already existing functions for checking if the uart is ready to read or write 🚀

Solves #836

[jannic]

The PR itself looks good.

However while reviewing it I noticed that we probably have a bug in our implementation of embedded_io::Write:

    fn write(&mut self, buf: &[u8]) -> Result<usize, Self::Error> {
        self.write_full_blocking(buf);
        Ok(buf.len())
    }

This will block if there's not enough FIFO space to hold the whole buf.

But that's not compatible with the definition of WriteReady: "If this returns true, it’s guaranteed that the next call to Write::write will not block."

As WriteReady can't know how many bytes the next call to write will contain, it can only give that guarantee if write returns without blocking as long as it can write a single byte. So write must not be implemented in terms of write_full_blocking.

(Fixed by #838)