Clean-up dependency for parsing of hs_ed25519_secret_key
Remove "bed25519" dependency in pkg/okey
and instead implement the crypto.Signer interface in this repo, maybe using:
Note that it is not possible to use crypto/ed25519
because the private key requires the seed, and what's stored in hs_ed25519_secret_key
is the expanded seed after hashing and setting a few bits.