Implement std::error::Error::source for bip152

During the recent merge of bip152 we ended up with a Rust 1.29 version
of the `error::Error`.

Implement `source` inline with our new MSRV.

src/util/bip152.rs

@@ -38,7 +38,16 @@ impl fmt::Display for Error {
 }
 
 #[cfg(feature = "std")]
-impl error::Error for Error {}
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match *self {
+            UnknownVersion | InvalidPrefill => None,
+        }
+    }
+}
 
 /// A [PrefilledTransaction] structure is used in [HeaderAndShortIds] to
 /// provide a list of a few transactions explicitly.