Halo2: Updates from latest master

[storojs72]

This PR updates halo2 branch of neptune to 0.8.1 as it is in master currently and fixes halo2 <-> master "behind" divergence.

It is required for updating halo2 branch of rust-fil-proofs eventually.

@vmx, @DrPeterVanNostrand, I guess, You are aware of master commits, so I would ask to review 9ac186e more carefully than others, as it contains merge conflicts fixes.

[vmx]

I think https://github.com/filecoin-project/neptune/tree/halo2-gpu is already doing most of this.

[storojs72]

I think https://github.com/filecoin-project/neptune/tree/halo2-gpu is already doing most of this.

Yeah, thanks for pointing. We can pick changes from your branch into halo2 instead, if you prefer. Anyway, it looks like that having single, unified branch with Halo2 "source of trust" is beneficial

[DrPeterVanNostrand]

Looks good to me

[vmx]

I'd prefer if we just force-push the halo2-gpu branch as halo2 as it has a linear history.

[storojs72]

@vmx , I don't mind using halo2-gpu. Actually, I didn't know that it exists with necessary changes until you mentioned. What if we use it but rename it to halo2 (and remove existing halo2)? Commit history will be still preserved, but there will be no confusion about single source of Halo2 work along the repositories. So, we can close this PR, remove halo2 branch, rename halo-gpu into halo2 and reference it in filecoin-project/rust-fil-proofs#1656. WDYT?

[vmx]

@storojs72 I've force-pushed halo2-gpu as halo2 (and pushed the former halo2 as halo2-jake in case he still needs it. Force-pushing halo2 branches isn't such a big deal as only our team is using it atm. I think this PR can be closed and the branch can be deleted once you've updated filecoin-project/rust-fil-proofs#1656.

[storojs72]

@vmx , thank you. I'm picking edited halo2 branch of neptune and testing filecoin-project/rust-fil-proofs#1656 right now.

[storojs72]

filecoin-project/rust-fil-proofs#1656 is working with updated halo2 branch, which seems to be the relevant source of Halo2 stuff (@vmx thank you!). So this PR is not needed anymore.