ZIP-211: Validate Disabling Addition of New Value to the Sprout Value Pool

[oxarbitrage]

Motivation

We need to add the disabled sprout pool consensus rule. Resolves #1564 when merged.

Specifications

https://zips.z.cash/zip-0211
https://zips.z.cash/protocol/protocol.pdf#joinsplitdesc

Solution

Implement the rule, add a test. The test can be improved.

Review

Reviewer Checklist

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

[teor2345]

I updated the title of this PR so it says "Disabling Addition of New Value to the Sprout Value Pool".

It is still possible to remove value from the sprout value pool.

[teor2345]

I think there's a bit of confusion in this PR about what ZIP-211 actually does.

Users can still remove value from the sprout pool. Only adding new value to the pool is disabled.

I also made some suggestions about code structure.

[teor2345]

We still need to:

• cover coinbase transactions in the tests, or skip them in the verifier
• make sure we're testing transactions with and without JoinSplits
• move the sprout check into the sprout verifier function

[teor2345]

• cover coinbase transactions in the tests, or skip them in the verifier

• make sure we're testing transactions with and without JoinSplits

This is done, just waiting on the refactor.

Some requested changes have been made, @jvff is reviewing the rest

[teor2345]

This is good enough for now.