feat(acir_field): Add little-endian byte serialization for FieldElement

[VolodymyrBg]

Description

Added a new to_le_bytes() method to FieldElement that provides native little-endian byte serialization. This complements the existing to_be_bytes() method and allows callers to choose their preferred endianness without manual byte reversal.

Changes:

• Added to_le_bytes() method that returns bytes in little-endian order
• Cleaned up documentation around byte ordering
• Added test case to verify correct endianness handling

Problem

Resolves #TODO

Summary

Added little-endian byte serialization support for FieldElement to provide more flexibility in byte order handling. Previously, the to_be_bytes() method was using little-endian serialization internally and then reversing the bytes. This PR adds a dedicated to_le_bytes() method to make the endianness handling more explicit and efficient.

Additional Context

The implementation:
Adds new to_le_bytes() method that returns bytes in little-endian order
Removes redundant comments from to_be_bytes()
Includes comprehensive test coverage for both endianness formats
Maintains backward compatibility with existing code

Documentation

[x] No documentation needed.

PR Checklist

[x] I have tested the changes locally.
Added new test case test_endianness() that verifies correct byte ordering
Verified existing tests pass
[x] I have formatted the changes with Prettier and/or cargo fmt on default settings.
Code follows Rust formatting guidelines
All new code is properly documented with comments

[github-actions]

Thank you for your contribution to the Noir language.

Please do not force push to this branch after the Noir team have started review of this PR. Doing so will only delay us merging your PR as we will need to start the review process from scratch.

Thanks for your understanding.

[VolodymyrBg]

@aakoshh Corrected

[VolodymyrBg]

@aakoshh Done

[aakoshh]

Thank you for adding this @VolodymyrBg 🙏

It looks good to me, but I'd like to defer to @TomAFrench about it, in case he remembers more context about this TODO.

FWIW I see one location where we're reversing after to_be_bytes, although I'm confused by the comment whether it should be big or little endian.

[TomAFrench]

This doesn't compile.

[aakoshh]

@VolodymyrBg can you run cargo fmt to format the source? You can configure your editor to do it for you on save. That's why it's failing: https://github.com/noir-lang/noir/actions/runs/13265342549/job/37106914185

[aakoshh]

Oh wait, it looks like fbfc367 truncated most of field_element.rs 👀

[VolodymyrBg]

Oh wait, it looks like fbfc367 truncated most of field_element.rs 👀

Really, corrected

[aakoshh]

Thanks. I'd like to ask though that you revert this branch to 445ac75 , apply the formatting fix there, then force push.

That's because if you look at the blame then it looks like you authored the whole thing, because it all comes from the commit that restored the lost lines. Compared that to what it looked like before a lot of the history is now buried.

[VolodymyrBg]

@aakoshh Done

[aakoshh]

@TomAFrench this seems ready apart from the circuit size report.

[VolodymyrBg]

@TomAFrench
@aakoshh

[aakoshh]

LGTM!