message: cargo fix --edition-idioms for ff
diff: #![allow(unused_imports)]
  -extern crate byteorder;
  -extern crate rand_core;
  +
  +
  #[cfg(feature = "derive")]
  #[macro_use]
  @@ -210,7 +210,7 @@ impl Error for PrimeFieldDecodingError {
  }
  impl fmt::Display for PrimeFieldDecodingError {
  - fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
  + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
  match *self {
  PrimeFieldDecodingError::NotInField(ref repr) => {
  write!(f, "{} is not an element of the field", repr)
  