Add more safety comments to `FromZeros::new_box_zeroed`

[AlexanderPortland]

Adds safety comments to two of the undocumented unsafe blocks in FromZeros::new_box_zeroed.

I tried to base their structure off of existing comments and am happy to do modifications if you have feedback!

Is it alright to trust that the std crate's default allocator will return an allocation of the right size and alignment for the given layout? I was looking through documentation to support this, but the std::alloc module says that the default global allocator is unspecified in most cases.

Towards #429

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[gemini-code-assist[bot]]

Summary of Changes

Hello @AlexanderPortland, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the safety documentation within the FromZeros::new_box_zeroed function. By adding comprehensive SAFETY comments to previously undocumented unsafe blocks, it clarifies the rationale behind these operations and ensures that the code adheres to Rust's safety guidelines, making it easier to understand and maintain.

Highlights

• Safety Comments Added: Introduced detailed SAFETY comments for two previously undocumented unsafe blocks within the FromZeros::new_box_zeroed function. These comments explain the invariants and justifications for the unsafe operations.
• Clarity for alloc::alloc::alloc_zeroed: A SAFETY comment was added for the alloc::alloc::alloc_zeroed call, clarifying that the layout's size is guaranteed to be non-zero before allocation, fulfilling its safety requirements.
• Clarity for Box::from_raw: A SAFETY comment was added for the Box::from_raw call, detailing that the allocation comes from the std crate's default Global allocator, is correctly aligned and non-null, and that T implements FromZeros ensuring a valid zeroed state.
• Removed clippy Allow Attributes: The #[allow(clippy::undocumented_unsafe_blocks)] attributes have been removed, as the unsafe blocks are now properly documented with SAFETY comments, addressing FIXME(#429).

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[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 92.33%. Comparing base (151c20f) to head (8d8bfce).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2847   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files          20       20           
  Lines        5794     5794           
=======================================
  Hits         5350     5350           
  Misses        444      444           

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