Improved Zeroize impl for BigInt's Sign (#39)

The `zeroize` crate defines a blanket impl of `Zeroize` for any type
with the `DefaultIsZeroes` marker trait, which replaces the value with
its `Default` using volatile writes with a memory fence.

This does require defining `Default` on `Sign`, but it seems like
`Sign::NoSign` is a fairly reasonable choice for a default.

src/bigint.rs

@@ -53,14 +53,15 @@ pub enum Sign {
     Plus,
 }
 
-#[cfg(feature = "zeroize")]
-impl Zeroize for Sign {
-    fn zeroize(&mut self) {
-        // TODO: Figure out how to better clear the sign.
-        *self = Sign::NoSign;
+impl Default for Sign {
+    fn default() -> Sign {
+        Sign::NoSign
     }
 }
 
+#[cfg(feature = "zeroize")]
+impl zeroize::DefaultIsZeroes for Sign {}
+
 impl Neg for Sign {
     type Output = Sign;