feat(iddfs): make IDDFS more efficient by using IndexSet internally

[samueltardieu]

This causes an API change as using an IndexSet requires the type to implement Hash.

[codspeed-hq]

CodSpeed Performance Report

Merging #554 will degrade performances by 45.76%

_{Comparing better-iddfs (c1e8c6c) with main (1ef8a9a)}

Summary

❌ 2 regressions
✅ 32 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	better-iddfs	Change
❌	fill-corner_to_corner_iddfs	23.1 ms	42.6 ms	-45.76%
❌	corner_to_corner_iddfs	8.2 ms	14 ms	-41.31%

[samueltardieu]

Clearly less efficient, closing.