Add a public static writer

Since `unsafe const` aren't allowed yet, we temorary add a custom version of Unique to make `Unique::new` const (instead of unsafe).

Cargo.toml

@@ -8,3 +8,4 @@ crate-type = ["staticlib"]
 
 [dependencies]
 rlibc = "*"
+spin = "*"

src/lib.rs

@@ -17,6 +17,7 @@
 #![no_std]
 
 extern crate rlibc;
+extern crate spin;
 
 mod vga_buffer;
 

src/vga_buffer.rs

@@ -1,8 +1,15 @@
 use core::ptr::Unique;
+use spin::Mutex;
 
 const BUFFER_HEIGHT: usize = 25;
 const BUFFER_WIDTH: usize = 80;
 
+pub static WRITER: Mutex<Writer> = Mutex::new(Writer {
+    column_position: 0,
+    color_code: ColorCode::new(Color::LightGreen, Color::Black),
+    buffer: unsafe{Unique::new(0xb8000 as *mut _)},
+});
+
 #[repr(u8)]
 pub enum Color {
     Black      = 0,