Always generate Arbitrary instances, not just with cfg(test)

src/signed/mod.rs

@@ -34,7 +34,6 @@ mod shift;
 #[macro_use]
 mod subtraction;
 
-#[cfg(test)]
 use quickcheck::{Arbitrary,Gen};
 use std::cmp::{Ord,Ordering,PartialOrd};
 use std::fmt;