Add a write_str method and print “Hello!”

src/vga_buffer.rs

@@ -74,6 +74,12 @@ impl Writer {
         }
     }
 
+    pub fn write_str(&mut self, s: &str) {
+        for byte in s.bytes() {
+        self.write_byte(byte)
+        }
+    }
+
     fn buffer(&mut self) -> &mut Buffer {
         unsafe{ self.buffer.as_mut() }
     }
@@ -88,5 +94,5 @@ pub fn print_something() {
         buffer: unsafe { Unique::new(0xb8000 as *mut _) },
     };
 
-    writer.write_byte(b'H');
+    writer.write_str("Hello!");
 }