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 19.93%

_{Comparing better-dfs (3464e58) with main (e32db8c)}

Summary

⚡ 3 improvements
❌ 1 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	4.5 ms	2.1 ms	×2.1
❌	fill-corner_to_corner_dijkstra	1.3 ms	1.7 ms	-19.93%
⚡	corner_to_corner_dfs	31.5 ms	1.7 ms	×18
⚡	corner_to_corner_dijkstra	1.9 ms	1.6 ms	+16.04%

[samueltardieu]

Code change is moot