feat: Add Sync to Backend in acvm

[skaunov]

Description

Problem*

Needed for threading in noir-lang/noir#2196.

Resolves none

Summary*

Add Sync to Backend in acvm

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[Savio-Sou]

Hey there, thanks for submitting the PR!

Could you elaborate a bit on how you plan on tackling noir-lang/noir#2196 and how would this help execute that plan?

[skaunov]

Obviously, it would be iterative for me, as I'm fairly new to the code-base. Pls, see noir-lang/noir@master...skaunov:noir:2196-Run-test-cases-for-nargo-test-in-parallel for the first step I have take. As you can see, the main idea is to feed test cases one-by-one to few thread running in parallel.

It was some time ago, I recollect that I tried to just unfold workspace to see how it would work, so there's mainly pasted code for index part. And after analyzing available options I couldn't see a way to run a test in a new thread without sending a reference to Backend to the thread.

If there's any other preferred way, just show how take it! X)