HMAC-based one-time passcodes
Provides implementations for both IETF RFCs:
* [4226](https://datatracker.ietf.org/doc/html/rfc4226): simple incrementing counter (HOTP)
* [6238](https://datatracker.ietf.org/doc/html/rfc6238): time-based counter (TOTP)
Custom HMAC-based one-time passcodes can be provided
by types implementing the [`ToBytes`] trait.