Allow to restore public key from private key

[IlyaGazman]

I am using your library for Java blockchain development: https://github.com/Confidence-coin/coco I was looking for the ability to generate a public key from a private key, eventually, I found this:

byte[] publicKey = new byte[32]; curve_sigs.curve25519_keygen(publicKey, ssk);

Can you make this code easier to find? For example, put it in Curve25519KeyPair.