Zebra: Implement ZIP-32 Sprout and Sapling key derivation

[dconnolly]

In order to match what zcashd currently does.

https://zips.z.cash/zip-0032

[dconnolly]

This is relevant to #265 and #266 work in progress.

[str4d]

Note that we haven't actually implemented Sprout HD derivation. We defined it for completeness, but @daira and I are considering removing it, and instead deprecating Sprout address generation in zcashd.

[str4d]

That said, if you do want to be able to generate Sprout address, HD generation from the same seed as Sapling addresses makes the most sense.

[mpguerra]

What are we missing here? Because I see that both #300 and #301 are done.

Can we close this one?

[teor2345]

Is there any verification-only work left in this ticket for NU5?

We won't need to derive addresses until we start work on zebra-client. And we might not want to generate sprout addresses anyway, because new sprout addresses can't receive any funds. (As of Canopy, transfers into the Sprout value pool are not allowed.)