crypto/ed25519/hd25519: Little-Endian -> Big-Endian
crypto/ed25519/hd25519: Little-Endian -> Big-Endian

Anticipates https://github.com/chain-engineering/chain/pull/1188

Closes chain/chainprv#1196.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain