feat: optimize FieldElement::num_bits

[asterite]

Description

Problem

No issue. While sampling a compilation it landed on FieldElement::num_bits so I was wondering if optimizing it would have any visible effect.

Summary

Before this PR num_bits would allocate the bytes in Vec, now I think there would be no allocations.

I didn't try it locally but it was a simple optimization so trying it here to see what CI says.

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.

[TomAFrench]

I don't think would show up in CI benchmarks as all of the allocations would be temporary and the time to count the numbers of bits in a field element would be insignificant when it comes to compilation.

Running heaptrack locally would show the difference, also you could add a criterion benchmark to track this.

[asterite]

Makes sense. I just benchmarked it locally and the method is like 5 times faster but, as you said, the numbers are too low for it to make a difference. So closing :-)

[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: 112ebc0	Previous: 8f20392	Ratio
AztecProtocol_aztec-packages_noir-projects_noir-protocol-circuits_crates_blob	62 s	51 s	1.22

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

CC: @TomAFrench

[asterite]

The benchmark said this is about 60% faster. Criterion doesn't show memory usage but the old method allocates memory while this new method doesn't, so I guess that's good too.