Sort the imports and merge them

[autquis]

This PR sorts the imports by cargo fmt and merges them.
Also, I removed some places that were using core and replaced them with ark_std. Please check if it is correct.

By the way, there is pub(crate) use ark_std::vec::Vec in lib.rs. I don't understand why we still have to import it when it is no-std? @mmagician

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• Targeted PR against the correct branch (main)
• 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

[Pratyush]

Sorry for the delay here, could you please rebase and then I can merge it!

[autquis]

Done! @Pratyush