fix: resolve easy TODOs and doc issues

[DaniPopes]

Closes #545 - const_eq counter-based implementation for ~200 cycle improvement on x86_64
Closes #349 - Fix incorrect panic doc on Matrix::from_u64 (said r0 < r1, should be r1 > r0)
Closes #323 - Remove stale // TODO: document algorithms from #![allow(missing_docs)] (unstable internal module, allow is intentional)

[codspeed-hq]

Merging this PR will improve performance by 11.04%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 3 improved benchmarks
✅ 383 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	most_significant_bits/4096/4096	28.9 µs	26.1 µs	+11%
⚡	to/f32/4096	29.1 µs	26.3 µs	+10.97%
⚡	to/f64/4096	29.3 µs	26.4 µs	+11.04%

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_{Comparing DaniPopes:dani/fix-easy-todos (062b991) with main (a21eb25)¹}

Footnotes

1. No successful run was found on main (062b991) during the generation of this report, so a21eb25 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩