verify with secret

[z33ky]

It seems to me that the API is missing a way to ergonomically verify an encoded hash that was used a secret.
argon2_verify() has no parameter for the secret, so argon2_verify_ctx() has to be used. decode_string() is not exposed however, so one has to reimplement decoding oneself, which is not ideal.

I propose either adding decode_string() to the public interface, or adding an argon2_verify_with_secret(const char *encoded, [...] const void *secret, const size_t secretlen, [...]) for this use case.
Having decode_string() be part of the public interface may also be useful for other use cases (dealing with ad), so I think that would be preferable.