Re-introduce integer reasoning when checking reasons in debug mode

[Dekker1]

This reintroduces functionality removed in 73f966b. The
problematic behaviour that changes of search order between debug and release builds is resolved by
ensuring the propagation queue remains unchanged while checking the reasons.

[codecov]

Codecov Report

Attention: Patch coverage is 91.52542% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/huub/src/solver/engine.rs	92.98%	4 Missing ⚠️
crates/fzn-huub/src/trace.rs	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
crates/huub/src/lib.rs	67.61% <100.00%> (-0.11%)	⬇️
crates/fzn-huub/src/trace.rs	44.33% <0.00%> (-0.53%)	⬇️
crates/huub/src/solver/engine.rs	80.49% <92.98%> (+0.49%)	⬆️

... and 15 files with indirect coverage changes

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[codspeed-hq]

CodSpeed Performance Report

Merging #167 will not alter performance

_{Comparing Dekker1:fix/reason_check (c0e6800) with develop (02d3bf5)}

Summary

✅ 14 untouched benchmarks