Relax the visibility for G2 ell coeffs and related algorithms

[weikengchen]

Description

This PR proposes to make it possible for a caller to use Arkworks-rs to obtain the ell coeffs and use it in other places.

Without this change, in the BitVM example here (https://github.com/BitVM/BitVM/blob/main/src/bn254/ell_coeffs.rs), one who wants to use Arkworks-rs's algorithm to get the ell coeffs would need to copy-paste the code.

This PR suggests that, since these ell coeffs are useful to the outside, it may be worthwhile to reveal them.

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• 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.
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

N/A:

• Wrote unit tests
• Updated relevant documentation in the code