[ecash-lib] Add HdNode for HD wallets

Summary:
Hierarchical deterministic wallets (BIP32) are the standard way to handle wallets in eCash.

This allows users to take a seed derived from a mnemonic and derive the private key of a given path.

Depends on D17612.

Test Plan: npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17614