Manually implement Debug for SerializedSignature

Currently we have an implementation of `Debug` (also used by `Display`)
for `Signature` that first converts the sig to a `SerializedSignature`
then prints it as hex.

We would like to have an implementation of `Debug` for
`SerializedSignature`, this cannot be derived because of the `data: [u8;
field]`. We can manually implement `Debug` for `SerializedSignature`
exactly as it is currently done for `Signature` and call this new
implementation from `Signature::fmt()`.

This code path is already tested in `lib.rs` in the test function
`signature_display`.

src/ecdsa/mod.rs

@@ -32,7 +32,19 @@ impl fmt::Debug for Signature {
 impl fmt::Display for Signature {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let sig = self.serialize_der();
-        for v in sig.iter() {
+        sig.fmt(f)
+    }
+}
+
+impl fmt::Debug for SerializedSignature {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+impl fmt::Display for SerializedSignature {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        for v in self.data.iter().take(self.len) {
             write!(f, "{:02x}", v)?;
         }
         Ok(())