More traits

We derived the Debug trait. It would be useful to implement a few more traits too.

use core::fmt::{self, Write};
impl Write for Uart {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        for c in s.as_bytes() {
            self.write_byte(*c);
        }
        Ok(())
    }
}
// SAFETY: `Uart` just contains a pointer to device memory, which can be
// accessed from any context.
unsafe impl Send for Uart {}

• Implementing Write lets us use the write! and writeln! macros with our Uart type.
• Run the example in QEMU with make qemu_minimal under src/bare-metal/aps/examples.