feat(stdlib): Add fallback implementation of SHA256 black box function

[Ethan-000]

Description

Problem*

Resolves

Summary*

Additional Context

PR Checklist*

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

[kevaundray]

Once we resolve the nits around styling etc, I think we can merge this in under a experimental feature flag, until we get someone to comb through the logic of sha256, so the PR does not get stale.

We would only need it to be on the method that converts sha256 blackbox functions into regular gates, so tests won't be affected since they will be switched on in acvm/stdlib unconditionally.

[kevaundray]

@Ethan-000 Please ping me when you want a re-review!

[Ethan-000]

I think its ready for a re-review :). I did all the suggested changes other than "using mutable reference for num_witness to avoid returning it"

[Ethan-000]

also added a sha256 prop test.

[kevaundray]

I think this is okay to merge since its isolated and not enabled -- great work!