Implement std::error::Error for all error types

Now that we have bumped the MSRV we can use `Error::source`. Implement
`std::error::Error` for all error types codebase wide.

src/key.rs

@@ -1321,7 +1321,11 @@ impl fmt::Display for InvalidParityValue {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl std::error::Error for InvalidParityValue {}
+impl std::error::Error for InvalidParityValue {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        None
+    }
+}
 
 impl From<InvalidParityValue> for Error {
     fn from(error: InvalidParityValue) -> Self {

src/lib.rs

@@ -372,25 +372,25 @@ impl fmt::Display for Error {
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 impl std::error::Error for Error {
-    #[allow(deprecated)]
-    fn cause(&self) -> Option<&dyn std::error::Error> {
-        match self {
-            Error::IncorrectSignature => None,
-            Error::InvalidMessage => None,
-            Error::InvalidPublicKey => None,
-            Error::InvalidSignature => None,
-            Error::InvalidSecretKey => None,
-            Error::InvalidSharedSecret => None,
-            Error::InvalidRecoveryId => None,
-            Error::InvalidTweak => None,
-            Error::NotEnoughMemory => None,
-            Error::InvalidPublicKeySum => None,
-            Error::InvalidParityValue(error) => Some(error),
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use Error::*;
+
+        match *self {
+            IncorrectSignature
+            | InvalidMessage
+            | InvalidPublicKey
+            | InvalidSignature
+            | InvalidSecretKey
+            | InvalidSharedSecret
+            | InvalidRecoveryId
+            | InvalidTweak
+            | NotEnoughMemory
+            | InvalidPublicKeySum => None,
+            InvalidParityValue(ref e) => Some(e),
         }
     }
 }
 
-
 /// The secp256k1 engine, used to execute all signature operations.
 pub struct Secp256k1<C: Context> {
     ctx: *mut ffi::Context,

src/scalar.rs

@@ -133,4 +133,8 @@ impl fmt::Display for OutOfRangeError {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl std::error::Error for OutOfRangeError {}
+impl std::error::Error for OutOfRangeError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        None
+    }
+}