Add methods for creating TOTP/HOTP from raw bytes and from base32-encoded strings

[jbg]

Currently the crate only accepts secrets in the form of Strings, which are valid UTF-8 by definition. The UTF-8 bytes of the string are then taken directly as the OTP secret. This is an unnecessary restriction on the possible secrets and significantly reduces the entropy in the resulting secret.

I added new methods for creating TOTP and HOTP from raw bytes and from base32-encoded strings.

Given the security shortcomings of the existing approach, I would also suggest that a release including these changes deprecate or remove the current new method and include a strong recommendation in the documentation/README to move to the from_bytes/from_base32 methods.

I used Option<T> as the return type for the from_base32 methods in order to keep these changes self-contained, but it might be worth adding an error type and using Result instead.

[messense]

Make this a pub(crate)?

[jbg]

Actually this method is quite useful for users of the crate. You can create the secret using securely-generated random bytes, and then you can get the base32 secret which is the standard format (other than QR codes) for giving the secret to users.