fix: Prevent negative zero

[jfecher]

Description

Problem*

Resolves #8498

Summary*

Prevents negative zero from being constructed in a SignedField wherever SignedField is used. This was only a known issue for the comptime interpreter but I thought it'd be safer to make these fields private so we could have the constructor check it everywhere.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 185810c	Previous: f4078e1	Ratio
test_report_AztecProtocol_aztec-packages_noir-projects_noir-contracts	118 s	97 s	1.22

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[rkarabut]

@jfecher looks like SignedField needs its fields to be public?

[jfecher]

@rkarabut those just need to be updated to use the accessor methods now instead.

Didn't know this PR didn't merge already, I'll fix it today.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Compilation Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 8e06b08	Previous: d03ec29	Ratio
rollup-base-public	15.66 s	12.42 s	1.26

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench