v0.0.7 - includes private key file encryption

This updates by adding a new CLI method: encryptkey to take a plaintext private key in WIF in a file (e.g. privkey) and requests a password and creates privkey.enc, encrypted with AES-GCM and using Argon2 for the password/salt.

The same is applied to createkeys and to prove methods - the password is passed via the RPC request, allowing the server to decrypt privkey.enc to execute the proving operation (or in the former case, to create the new privkey.enc file).