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]

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