Spending Key Derivation in the `zcashd` wallet

Over Zcash's history, different wallets have used many different approaches to generating keys. Today, we have standardized around HD derivation based on BIP 39 seed phrases, but many older wallets used different strategies, including creating keys from system randomness. In particular, the zcashd wallet initially generated keys from system randomness, then for Sapling implemented HD derivation according to ZIP 32 but used a seed directly generated from system randomness, not BIP 39 seed phrases.

Other derivation schemes that need to be documented:

• Zecwallet Lite incorrectly used derivation of fresh accounts to get new Sapling addresses when it should have been generating diversified addresses.