message: Use `digest` at root level
diff: //! To be used for / with already hashed values, just to comply with the Digest contract.
  //!
  //! Adapted from `sha2` [sha256.rs](https://github.com/RustCrypto/hashes/blob/master/sha2/src/sha256.rs)
  -use sha2::digest::consts::U32;
  -use sha2::digest::generic_array::GenericArray;
  -use sha2::digest::{FixedOutputDirty, Output, Reset, Update};
  +use digest::consts::U32;
  +use digest::generic_array::GenericArray;
  +use digest::{FixedOutputDirty, Output, Reset, Update};
  /// The 256-bits identity container
  #[derive(Clone)]
  