Let ark-ff specify the same num-bigint version as the rest of the repos

[weikengchen]

Description

This is related to #316 but is not expected to be a solution.

There is a pending "unstable feature" in Rust 1.56 beta that requires num-bigint to be bumped up to 0.4.2 or higher to be able to compile.

My local test seems to show that the compiler would always retrieve the 0.4.2 version (since arkwork-rs never uses Cargo.lock). So this seems to be an issue for downstream applications to handle.

Nevertheless, we can improve the current code: only the algebra and r1cs-std repos use num-bigint, and ark-ff is the only one that is specifying 0.4.0, while the rest are all using 0.4.

This PR changes what ark-ff specifies in the Cargo.toml to 0.4, thereby keeping it consistent with the rest.

This does not closes #316. We will see whether more changes will be needed due to potential Rust changes.

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Re-reviewed Files changed in the Github PR explorer

N/A:

• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md

[vlopes11]

LGTM. Generally speaking its a good thing to use only major/minor, but we commit that num-bigint follows semver good practices.

I'll in parallel check their history and will come back to you in case historically they make breaking changes as patches

[mmagician]

Looks good to me 👍🏼

[weikengchen]

Some additional notes before I merged:

1. About semver (major/minor/patch), I agree that keeping major/minor should be sufficient for our cases.

2. There seem to be no breaking changes between 0.4.0 and 0.4.2 (https://docs.rs/crate/num-bigint/0.4.2/source/RELEASES.md). The main commit that is related to the issue is rust: use explicitily Integer::div_ceil rust-num/num-bigint#219.