In the latest `parity-crypto` release (upcoming 0.4), the aes GCM features were removed (done to remove the dependency on `ring`).
This PR adds the bare minimum crypto needed for Whisper directly to the crate itself and as those were the only features needed from `parity-crypto`, removes the dependency on that crate altogether.

Reverts using NonZeroU32 (introduced [here](b347599)).

* master:
  docs: Add ProgPoW Rust docs to ethash module (#10653)
  fix: Move PR template into .github/ folder (#10663)
  docs: Add PR template (#10654)
  Trivial journal for private transactions (#10056)
  fix(compilation warnings) (#10649)
  [whisper] Move needed aes_gcm crypto in-crate (#10647)

* master:
  Remove support for hardware wallets (#10678)
  [CI] allow cargo audit to fail (#10676)

* master:
  Don't panic if extra_data is longer than VANITY_LENGTH (#10682)
  docs: evmbin - Update Rust docs (#10658)
  Remove annoying compiler warnings (#10679)
  Reset blockchain properly (#10669)