Implement Backend for AVX512Backend

[spapinistarkware]

This change is 

[spapinistarkware]

• Implement Backend for AVX512Backend #491 👈
• Make prove() generic in Backend #490 : 1 other dependent PR (#492 )
• CommitmentSchemeProver generic in backend #489
• Fri AVX ops #563
• AVX quotients #488
• Remove old merkle #531 : 1 other dependent PR (#486 )
• FRI using simple merkle #530
• Dumb down FRI #529
• avx merkle #528
• simple merkle benchmark #527
• Use simple merkle tree #526
• merkle intermediate layers #548
• dev

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 93.43%. Comparing base (6a9223d) to head (504018a).

Files	Patch %	Lines
src/core/backend/avx512/accumulation.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@                               Coverage Diff                               @@
##           spapini/03-16-make_prove_generic_in_backend     #491      +/-   ##
===============================================================================
- Coverage                                        93.50%   93.43%   -0.08%     
===============================================================================
  Files                                               63       64       +1     
  Lines                                             7881     7903      +22     
  Branches                                          7881     7903      +22     
===============================================================================
+ Hits                                              7369     7384      +15     
- Misses                                             454      461       +7     
  Partials                                            58       58              

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[andrewmilson]

Reviewable status: 0 of 7 files reviewed, 1 unresolved discussion (waiting on @spapinistarkware)

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src/core/backend/avx512/mod.rs line 139 at r4 (raw file):

impl SecureColumn<AVX512Backend> {
    pub fn packed_at(&self, vec_index: usize) -> PackedQM31 {

Why removed the Safety and unsafe?