Validate field elements to ensure 254-bit constraint

[albertyosef]

Description:
This PR addresses field element validation to ensure proper bit-width constraints are enforced. Previously, we accepted field elements that fit within 256 bits without properly validating against the BN254 curve's 254-bit constraint.

Key Changes:

• Replace BLS12-377 with BN254 curve for proper field element handling
• Introduce fieldElementBitWidth constant (254 bits) for explicit validation
• Ensure field elements are properly validated against BN254's field size
• Add test cases to verify 254-bit constraint enforcement

Technical Details:

• Switch from bls12-377/fr to bn254/fr for correct field arithmetic
• Explicit validation that field elements are within 254 bits
• ByteWidth calculation now uses (fieldElementBitWidth+7)/8 for clarity
• Test cases cover:
  • Valid: 253-bit numbers
  • Boundary: 254-bit numbers
  • Reduction: Numbers > 254 bits (which get reduced mod r)

Testing:

• Added comprehensive test cases for field element validation
• Verified proper reduction behavior for the BN254 field
• All existing tests pass with the new validation

Fixes #282

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[albertyosef]

@DavePearce hey, could you please review the changes, and check if everythings alright !

[DavePearce]

Hi @albertyosef --- sorry I'm not following what you are trying to achieve here? Do you have some reason for wanting the BN254 curve?

[albertyosef]

@DavePearce I apologize for the confusion. I misinterpreted the original issue (#282) about field element validation. I incorrectly assumed we needed to switch to BN254 when the actual requirement was just to ensure proper 254-bit constraint validation for field elements.

Let me revise the PR to focus on the actual issue:

1. Implement proper validation for 254-bit field elements
2. Add the fieldElementBitWidth constant for explicit validation
3. Add test cases to verify the constraints

I'll remove the curve switching changes and submit an updated version that only addresses the field element validation. Thank you for catching this misunderstanding.

[albertyosef]

Hi @DavePearce, apologies for the earlier confusion regarding the BN254 curve. I've revised the PR to focus solely on the field element validation as per issue #282. The changes now correctly implement the 254-bit constraint, include the fieldElementBitwidth constant, and have corresponding test cases. Thanks for your understanding! Please do guide me if there is any changes required.