der: use core::error::Error (#1553)

Allows the `Error` trait to be used on `no_std` platforms

der/src/error.rs

@@ -70,8 +70,7 @@ impl Error {
  }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for Error {}
+impl core::error::Error for Error {}
 
 impl fmt::Display for Error {
  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {