perf: replace HashMap and HashSet with FxHashMap and FxHashSet
- Improved performance by using rustc-hash's FxHashMap and FxHashSet.
- Limited usage to internal library components since the algorithm is non-cryptographic.
refactor: unify image segmentation logic using LabelImage, SegmentMetadata, and Builder structs
Extracted common logic for superpixel-based image segmentation into shared structures (`LabelImage`, `SegmentMetadata`, `Builder`).
Improved code maintainability, and reduced duplication across SLIC, SNIC, Kmeans, DBSCAN, and FastDBSCAN algorithms.