Simplify `PolynomialCommitment` trait: remove the generic on `S: CryptographicSponge`

[mmagician]

Description

Quality of life improvement for downstream users of the library.

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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.
• [ ] Wrote unit tests
• [ ] Updated relevant documentation in the code
• [ ] Added a relevant changelog entry to the Pending section in CHANGELOG.md
• [ ] Re-reviewed Files changed in the Github PR explorer

[autquis]

I pushed this to make the tests pass. I can also open another PR to the master for this and then you can rebase this one on that; let me know.

[mmagician]

@autquis Thank you!

[mmagician]

It's a bit annoying with that stable/nightly discrepancy on imports

[mmagician]

@autquis I had to undo your changes, as no-std was failing. In the end I ended up following the same strategy as https://github.com/arkworks-rs/algebra/pull/790/files#diff-6035356f5adc30af6393d5492dbc1d9d8ec187272030fdd95709b1b30f1275fc

[autquis]

I wonder, isn't it clenaer if we use #[cfg(not(feature = "std"))] for these imports?

[mmagician]

That's worth exploring yes

[mmagician]

The checks in #130 are failing. They've already been addressed here, we can either try to cherry pick the relevant commit to Hyrax, or merge this PR first and adapt Hyrax accordingly to remove the generic S. I'm in favor of the latter @Pratyush

[autquis]

I added ark-std to the patch due to arkworks-rs/std#48 CI fails because of the same issue of imports in crypto-primitive. But poly-commit is fine now.

Also, I am doing a bit of cleaning up in the order of imports, as we are touching imports anyway.

[autquis]

This PR depends now on arkworks-rs/crypto-primitives#142 Once that is merged, we can update Cargo.toml.

[autquis]

The PR is ready to be merged. @z-tech @Pratyush @weikengchen

[autquis]

Ping! Once this gets merged, I can move to the other PRs (Hyrax, Brakedown) and prepare them for being merged.

[autquis]

Another ping. @weikengchen @Pratyush @z-tech

Also, it seems the cfg(not(feature("std"))) is no longer needed (rustc stable got upgraded). Once you review this, I'll remove them, and then, merge the PR, please. I'm not changing it now.

[autquis]

Gentle reminder @Pratyush