Allow dead code in span.rs

There's some API design that I'm happy with so far but gives us some
unused code. I don't want to get rid of it so we'll just allow it for
now.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/span.rs

@@ -1,3 +1,5 @@
+#![allow(dead_code)]
+
 use std::fmt::{self, Debug};
 use std::rc::Rc;