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:

• https://pkg.go.dev/filippo.io/edwards25519

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.