Sprout note commitment trees

[upbqdn]

Motivation

Zebra needs Spout note commitment trees.

Designs

https://github.com/ZcashFoundation/zebra/blob/a1cec27871f22dd1d8a8440e4fd4e7b8ff111374/book/src/dev/rfcs/drafts/0005-treestate.md

https://zips.z.cash/protocol/protocol.pdf#merkletree

Solution

This PR follows the implementation of the note commitment trees for Sapling.

Closes #2484.

Review

@dconnolly can review.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

Follow Up Work

This PR is a prerequisite for #2485.

[upbqdn]

I want to add more tests from Sapling once I solve the currently failing test.

[conradoplg]

Looks good, great work searching for those transactions with JoinSplits. (I found it interesting that it took that long for them to appear in the blockchain).

I just added an optional suggestion that we can do in a separate ticket if you prefer.

[upbqdn]

I found it interesting that it took that long for them to appear in the blockchain.

That was surprising to me as well. What I did there was a manual binary search for the first anchor different from the genesis one, which is the hash of an empty note commitment tree anyway. I felt the mighty power of logarithms when I was doing the search. :D

[teor2345]

Only minor changes since the last approval