Add a basic writer type to print single bytes

It prints always to the last line and shifts lines up on newline or line wrap. To easily shift lines up, we make the ColorCode and ScreenChar types Copy.

src/lib.rs

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 #![feature(no_std, lang_items)]
-#![feature(const_fn)]
+#![feature(const_fn, unique)]
 #![no_std]
 
 extern crate rlibc;