This is largely based on the existing X25519 and X448 serializers - but
a few adjustments were necessary so that we can identify what type of key
we are using. Previously we used the keylen for this but X25519 and
ED25519 have the same keylen.
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11272)
