Put Error impl block below Display

In an effort to be uniform throughout the codebase; put the
`std::error::Error` impl block below the `Display` impl block.

Author: tcharding

src/util/bip158.rs

@@ -74,10 +74,6 @@ pub enum Error {
     Io(io::Error),
 }
 
-#[cfg(feature = "std")]
-#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for Error {}
-
 impl Display for Error {
     fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
         match *self {
@@ -87,6 +83,10 @@ impl Display for Error {
     }
 }
 
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl ::std::error::Error for Error {}
+
 impl From<io::Error> for Error {
     fn from(io: io::Error) -> Self {
         Error::Io(io)