perf: Reuse caches for nth index selectors

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (9024970) 85.17% compared to head (8ffea7f) 84.87%.

Files	Patch %	Lines
css-inline/src/html/document.rs	12.50%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #278 +/- ## ========================================== - Coverage 85.17% 84.87% -0.31% ========================================== Files 18 18 Lines 1693 1699 +6 ========================================== Hits 1442 1442 - Misses 251 257 +6 ```

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #278 will improve performances by 25.43%

_{Comparing dd/use-more-cache (8ffea7f) with master (9024970)}

Summary

⚡ 1 improvements ✅ 5 untouched benchmarks

Benchmarks breakdown

	Benchmark	master	dd/use-more-cache	Change
⚡	big_page	10.8 s	8.6 s	+25.43%