ed25519: ed25519 cryptographic signature support

This module implements the ed25519 signature algorithm, as defined by RFC 8032.

Do not use the same secret key for both key exchanges and signatures. The public
keys are different and revealing both may leak information.

This is a low-level module which implements cryptographic primitives. Direct use
of cryptographic primitives is not recommended for non-experts, as incorrect use
of these primitives can easily lead to the introduction of security
vulnerabilities. Non-experts are advised to use the high-level operations
available in the top-level [[crypto::]] module.

Be advised that Hare's cryptography implementations have not been audited.
