Use gnutls instead of gcrypt

Hi! I am pleased to see the introduction of an alternative cryptography library: the web subtle crypto API. The cleanup process makes it even easier to use Gnutls instead of Gcrypt. Gnutls is a cryptography library which is widely used in Guile projects (for instance, it empowers the (web client) standard module to reach the HTTPS world), and is already used as a dependency in Goblins. It is quite easy to switch from Gcrypt to Gnutls. Unfortunately, the hex-encode function is not yet present in the gnutls-guile version in guix, so it is not perfectly ready yet.

Best regards,

Vivien