add implementation of write_char for VgaBuf

boot1/src/vga.rs

@@ -81,4 +81,9 @@ impl fmt::Write for VgaBuf {
     self.vga_write_str(s);
     Ok(())
   }
+  
+  fn write_char(&mut self, c: char) -> fmt::Result {
+    self.vga_write_char(c);
+    Ok(())
+  }
 }