chore: update unsafe function and improve documentation and safety notes

[prestwich]

Closes #550
Supersedes #551
Addresses rustsec/advisory-db#2538

Solution

Review safety of unstable crate API functions. Ensure that functions with COUs that may result in UB are marked as unsafe, and have clear COU documentation

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[codspeed-hq]

CodSpeed Performance Report

Merging #552 will degrade performance by 25.91%

_{Comparing prestwich/rustsec-safety (51fdde2) with main (17c9b3e)}

Summary

⚡ 8 improvements
❌ 14 regressions
✅ 364 untouched

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
⚡	div_rem/4096/2048	686 µs	609.5 µs	+12.55%
⚡	fmt/binary/128	7.3 ms	6.6 ms	+10.48%
⚡	log/128	8.8 ms	7.4 ms	+18.88%
❌	log/192	11.8 ms	14 ms	-15.65%
❌	log/256	14.4 ms	16.5 ms	-13.15%
❌	log/384	21.1 ms	24 ms	-12.02%
⚡	fmt/hex/128	3 ms	2.4 ms	+26.18%
⚡	fmt/hex/64	1.5 ms	1.4 ms	+12.29%
⚡	most_significant_bits/4096/4096	29 µs	26 µs	+11.51%
❌	overflowing_pow/256	2 ms	2.5 ms	-20.55%
❌	overflowing_pow/192	1.1 ms	1.5 ms	-25.91%
⚡	overflowing_pow/128	534.8 µs	460.4 µs	+16.16%
❌	overflowing_pow/512	9 ms	10.4 ms	-13.18%
❌	overflowing_pow/4096	1.8 ms	2.2 ms	-20.99%
❌	overflowing_pow/384	4.5 ms	5.4 ms	-17.22%
⚡	pow/4096	2.2 ms	2 ms	+10.49%
❌	root/127/192/192	15.7 ms	18.6 ms	-15.63%
❌	root/127/512/512	27.7 ms	32.5 ms	-14.63%
❌	root/127/256/256	11.9 ms	13.6 ms	-12.77%
❌	root/127/384/384	13.7 ms	15.4 ms	-11.28%
...	...	...	...	...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

[prestwich]

cc @DaniPopes in case you want to look into why benches have changed

[DaniPopes]

Likely due to new Rust stable version, I also saw this on a CI change not that long ago