Implement RFC5: State updates Chain type

[yaahc]

No description provided.

[teor2345]

Looks good, but needs a few tweaks and bug fixes.

I don't understand enough about transactions to review that part of the code, let's get @dconnolly or @hdevalence to review it as well.

In particular, I don't know if every post-Sapling block is guaranteed to have a Sprout and Sapling anchor. If they don't, that will affect this PR:
#902 (review)

There are also a few outstanding design issues, which were part of my final review of the RFC, but didn't get included. I don't think they affect this PR, but please check:
#1090

[teor2345]

Looks great!

I'm not sure if you'd like to merge now, or do some more work, and then merge.

I think it's ok to merge the partial PR, as long as someone else has reviewed the transaction parts.

[yaahc]

I want to get the two proptests that henry mentioned implemented before merging, just having a bit of trouble figuring out how to make arbitrary fake chains 😅

[teor2345]

I want to get the two proptests that henry mentioned implemented before merging, just having a bit of trouble figuring out how to make arbitrary fake chains 😅

Let me know if you need help with that - you might want to start at genesis, and then add various forks.

[teor2345]

Just following up on the sprout and sapling roots - it doesn't look like we'll need to change the RFC design or implementation:
https://github.com/ZcashFoundation/zebra/pull/902/files#r493427948

[teor2345]

My original utxo_is_available was meant for contextual verification, not prospective verification. Since a chain can fork at any non-finalized block, I don't think we should check spent_utxo here.

Instead, we should check spent_utxo using the original utxo_is_available logic during contextual verification.

Suggested change

	- (non-finalized) if any `Chains` contain an `OutPoint` in their `created_utxos` and not their `spent_utxo` get the `transparent::Output` from `OutPoint`'s transaction
	- (finalized) else if `OutPoint` is in `utxos_by_outpoint` return the associated `transparent::Output`.
	- (non-finalized) if any `Chains` contain `OutPoint` in their `created_utxos`, get the `transparent::Output` from `OutPoint`'s transaction
	- (finalized) else if `OutPoint` is in `utxos_by_outpoint` return the associated `transparent::Output`.

[yaahc]

applied this in the test PR to avoid needing a new test and approval

[teor2345]

I think we're looking pretty good to merge here.

We just need a tweak to the state design, and maybe we also want to get the separate tests PR working?