Add a VGA buffer module skeleton

It contains basic types for colors, screen characters, and the buffer itself.

src/lib.rs

@@ -13,10 +13,13 @@
 // limitations under the License.
 
 #![feature(no_std, lang_items)]
+#![feature(const_fn)]
 #![no_std]
 
 extern crate rlibc;
 
+mod vga_buffer;
+
 #[no_mangle]
 pub extern fn rust_main() {
     // ATTENTION: we have a very small stack and no guard page