Add serde impl for EcdsaSig

Author: dr-orlovsky

src/util/ecdsa.rs

@@ -418,6 +418,7 @@ impl<'de> ::serde::Deserialize<'de> for PublicKey {
 
 /// An ECDSA signature with the corresponding hash type.
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct EcdsaSig {
     /// The underlying ECDSA Signature
     pub sig: secp256k1::ecdsa::Signature,