Deterministically derive an Ed25519 key pair from Seed, 32
arbitrary bytes. Seed can be chosen at random using
crypto_n_random_bytes/2 or derived from input keying material
using crypto_data_hkdf/4.
KeyPair is a hexadecimal atom denoting the key pair in
PKCS#8 v2 format (RFC 5958, RFC 8410), the format also used by
openssl genpkey -algorithm ed25519. It contains the private key
and must be kept absolutely secret. It can be used for signing
with ed25519_sign/4, and its public key is obtained with
ed25519_keypair_public_key/2.
