Go Implementation of do255e and do255s

This is a plain Go implementation of do255e and do255s. It is considered secure; all relevant functions should be constant-time on modern architectures.

The do255e and do255s packages provide APIs for:

• the prime order group itself;
• scalars (i.e. integers modulo the primer order of the group);
• high-level functionalities: key pair generation, key exchange (Diffie-Hellman), signature generation and verification, and hash-to-curve.

The implementation requires Go-1.12 or later. It is portable; there is no assembly or other architecture-dependent feature.