Added public API to support BIP32-Ed25519 style keys.

[nalyd88]

Added public APIs to the Ed25519 Donna code that may be used to support Extended Ed25519 keys, i.e., BIP32-Ed25519 keys. BIP32-Ed25519 keys are used for hierarchical-deterministic (HD) wallets and are defined in the paper "BIP32-Ed25519 Hierarchical Deterministic Keys over a Non-linear Keyspace" by Khovratovich and Law. https://raw.githubusercontent.com/input-output-hk/adrestia/master/docs/static/Ed25519_BIP.pdf

The added API functions use existing internal functionality i.e., no new cryptographic functionality was added. Test code was added to validat7.cpp.