First whack at modular inverses.

src/cryptonum/signed.rs

@@ -20,6 +20,10 @@ impl SCN {
         self.value.is_zero()
     }
 
+    pub fn is_negative(&self) -> bool {
+        self.negative
+    }
+
     pub fn from_str(x: &str) -> SCN {
         if x.get(0..1) == Some("-") {
             SCN{ negative: true, value: UCN::from_str(&x[1..]) }
@@ -59,6 +63,18 @@ define_signed_into!(SCN, i16, u16);
 define_signed_into!(SCN, i32, u32);
 define_signed_into!(SCN, i64, u64);
 
+impl From<UCN> for SCN {
+    fn from(x: UCN) -> SCN {
+        SCN{ negative: false, value: x }
+    }
+}
+
+impl Into<UCN> for SCN {
+    fn into(self) -> UCN {
+        self.value
+    }
+}
+
 //------------------------------------------------------------------------------
 //
 //  Comparisons