error: drop 'source' impl

We can either write an underlying error's display string in our
own Display impl OR we can return it as a source. We can't do
both, otherwise we wind up with repeating error messages when using
today's error helper libraries (like anyhow).

Since the error types are either exposed or already offer a way
to get the underlying error, we just drop the 'source' impl.

src/error.rs

@@ -133,19 +133,7 @@ impl From<Error> for io::Error {
  }
 }
 
-impl StdError for Error {
- fn source(&self) -> Option<&(dyn StdError + 'static)> {
- match *self.0 {
- ErrorKind::Io(ref err) => Some(err),
- ErrorKind::Utf8 { ref err, .. } => Some(err),
- ErrorKind::UnequalLengths { .. } => None,
- ErrorKind::Seek => None,
- ErrorKind::Serialize(_) => None,
- ErrorKind::Deserialize { ref err, .. } => Some(err),
- _ => unreachable!(),
- }
- }
-}
+impl StdError for Error {}
 
 impl fmt::Display for Error {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
@@ -333,11 +321,7 @@ impl<W> IntoInnerError<W> {
  }
 }
 
-impl<W: std::any::Any> StdError for IntoInnerError<W> {
- fn source(&self) -> Option<&(dyn StdError + 'static)> {
- self.err.source()
- }
-}
+impl<W: std::any::Any> StdError for IntoInnerError<W> {}
 
 impl<W> fmt::Display for IntoInnerError<W> {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {