Added directfs

Added a very rudimentary file I/O system suitable for experimenting
with the language further. A better one will be designed when we have
sensible error management.

src/systems/stl/str.rs

@@ -65,7 +65,7 @@ expr=x in
     let mut graphs = s.as_str().graphemes(true);
     let a = graphs.by_ref().take(i as usize).collect::<String>();
     let b = graphs.collect::<String>();
-    Ok(tuple(vec![a.into(), b.into()]))
+    Ok(tuple([a.into(), b.into()]))
   }
 }