feat(dfs): make DFS more efficient by using IndexMap rather than Vec

[samueltardieu]

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

[codspeed-hq]

CodSpeed Performance Report

Merging #553 will degrade performances by 11.93%

_{Comparing better-dfs (884e7d5) with main (1ef8a9a)}

Summary

⚡ 2 improvements
❌ 2 regressions
✅ 30 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	better-dfs	Change
⚡	fill-corner_to_corner_dfs	15.1 ms	7.7 ms	+94.57%
❌	fill-corner_to_corner_dijkstra	7.2 ms	8.1 ms	-11.93%
⚡	corner_to_corner_dfs	187.1 ms	8.1 ms	×23
❌	no_path_dijkstra	7.1 ms	8.1 ms	-11.76%