Cleanup usage of unsafe blocks

[justsmth]

Description of changes:

• Cleanup our usage of unsafe blocks.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.82353% with 18 lines in your changes missing coverage. Please review.

Project coverage is 92.59%. Comparing base (c358484) to head (c18b133).
Report is 55 commits behind head on main.

Files	Patch %	Lines
aws-lc-rs/src/ec.rs	77.14%	0 Missing and 8 partials ⚠️
aws-lc-rs/src/rsa/key.rs	72.22%	0 Missing and 5 partials ⚠️
aws-lc-rs/src/agreement.rs	81.81%	0 Missing and 2 partials ⚠️
aws-lc-rs/src/ec/key_pair.rs	66.66%	0 Missing and 2 partials ⚠️
aws-lc-rs/src/rsa/encoding.rs	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #499      +/-   ##
==========================================
- Coverage   95.80%   92.59%   -3.21%     
==========================================
  Files          61       66       +5     
  Lines        8143     8814     +671     
  Branches        0     8814    +8814     
==========================================
+ Hits         7801     8161     +360     
- Misses        342      396      +54     
- Partials        0      257     +257     

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