* [whisper] Move needed aes_gcm crypto in-crate
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.
* Upgrade to parity-crypto 0.4
Reverts using NonZeroU32 (introduced [here](b347599cf7)).
* Check for 0 in `args.arg_keys_iteration`
* Use beta.4
* parity-crypto 0.4.0 is released