Implement printf (very simple version)

platform/src/lib.rs

@@ -17,3 +17,22 @@ mod sys;
 mod sys;
 
 pub mod types;
+
+use core::fmt;
+
+use types::c_int;
+
+pub struct FileWriter(pub c_int);
+
+impl FileWriter {
+    pub fn write(&mut self, buf: &[u8]) {
+        write(self.0, buf);
+    }
+}
+
+impl fmt::Write for FileWriter {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.write(s.as_bytes());
+        Ok(())
+    }
+}