Merge rust-bitcoin/rust-secp256k1#441: Derive Hash for Signature

ef7f1972a7e412e8b7a2e83f1c6eca0beed6559b Derive Hash for Signature (Tobin C. Harding)

Pull request description:

  In preparation for deriving `Hash` in miniscript, derive `Hash` on the `ecdsa::Signature`.

  ref: rust-bitcoin/rust-miniscript#226

ACKs for top commit:
  apoelstra:
    ACK ef7f1972a7e412e8b7a2e83f1c6eca0beed6559b
  elichai:
    ACK ef7f1972a7e412e8b7a2e83f1c6eca0beed6559b

Tree-SHA512: 7313f59971444ae18611adbafe86a09478eddd7357f2b7f3ad3bb1761609b6358b156975086f6c318eb2777018b7b2f44386321108939acbcf2d0a522e7e208e

Author: chain-forgexcr45

src/ecdsa/mod.rs

@@ -16,7 +16,7 @@ pub use self::recovery::{RecoveryId, RecoverableSignature};
 use SECP256K1;
 
 /// An ECDSA signature
-#[derive(Copy, Clone, PartialEq, Eq)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct Signature(pub(crate) ffi::Signature);
 
 /// A DER serialized Signature