uucore: switch to BigDecimal::powi implementation

[xtqqczze]

Related: akubera/bigdecimal-rs#148

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #9957 will improve performance by 10.79%

_{Comparing xtqqczze:bigdecimal-powi (c442fda) with main (1eea517)}

Summary

⚡ 9 improvements
✅ 127 untouched
⏩ 24 skipped¹

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
⚡	sort_case_insensitive[500000]	276.7 ms	265.9 ms	+4.07%
⚡	sort_mixed_data[500000]	328.1 ms	317.3 ms	+3.42%
⚡	sort_ascii_utf8_locale	22.2 ms	20.1 ms	+10.79%
⚡	sort_numeric_utf8_locale	23.8 ms	22.8 ms	+4.49%
⚡	sort_case_sensitive[500000]	174 ms	163.1 ms	+6.64%
⚡	sort_ascii_only[500000]	353.8 ms	343 ms	+3.16%
⚡	sort_accented_data[500000]	362.6 ms	351.8 ms	+3.08%
⚡	sort_reverse_locale[500000]	361.8 ms	350.9 ms	+3.09%
⚡	sort_ascii_c_locale	22.4 ms	20.3 ms	+10.69%

Footnotes

1. 24 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩