Implement fmt::Write trait for Writer

src/vga_buffer.rs

@@ -1,4 +1,5 @@
 use core::ptr::Unique;
+use core::fmt;
 use spin::Mutex;
 
 const BUFFER_HEIGHT: usize = 25;
@@ -78,6 +79,15 @@ impl Writer {
     }
 }
 
+impl fmt::Write for Writer {
+    fn write_str(&mut self, s: &str) -> ::core::fmt::Result {
+        for byte in s.bytes() {
+          self.write_byte(byte)
+        }
+        Ok(())
+    }
+}
+
 #[derive(Clone, Copy)]
 struct ColorCode(u8);