Use GMP factor removal for decimals

[terror]

Replaces repeated factor-removal loops in decimal conversion with GMP-backed remove_factor_mut. Stack-backed MiniInteger factors avoid additional heap allocations.

Criterion results for non-terminating denominators with large powers of 2 and 5:

Denominator	Before	After	Speedup
3 * 2^100	2.00 us	636 ns	3.1x
3 * 2^1000	36.0 us	698 ns	51.6x
3 * 5^100	3.85 us	1.07 us	3.6x
3 * 5^1000	181 us	2.93 us	61.7x

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.45%. Comparing base (bba9769) to head (70d3b13).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
- Coverage   78.49%   78.45%   -0.04%     
==========================================
  Files          29       29              
  Lines        3520     3514       -6     
==========================================
- Hits         2763     2757       -6     
  Misses        757      757              

Flag	Coverage Δ
unit	78.45% <100.00%> (-0.04%)	⬇️

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