Add a quick test to ensure that our GCD algorithm works.

src/cryptonum/signed.rs

@@ -8,7 +8,7 @@ use std::ops::*;
 #[derive(Clone,Debug,PartialEq,Eq)]
 pub struct SCN {
     negative: bool,
-    value: UCN
+    pub(crate) value: UCN
 }
 
 impl SCN {
@@ -308,4 +308,11 @@ mod test {
             (- (&a + &b)) == ((- &a) + (- &b))
         }
     }
+
+    quickcheck! {
+        fn egcd_works(a: SCN, b: SCN) -> bool {
+            let (d, x, y) = a.clone().egcd(b.clone());
+            ((a * x) + (b * y)) == d
+        }
+    }
 }