Enhance SlotId and SlotArena for improved stability and performance (#29)

- Updated `SlotId` to include a generation counter, preventing stale handle access and ensuring safe reuse of indices.
- Modified `SlotArena` to manage a separate generations vector, allowing for efficient tracking of slot reuse and enhancing the safety of `get`, `remove`, and `contains` operations.
- Improved documentation to clarify the behavior of `SlotId` and its relationship with `SlotArena`, including examples demonstrating the new generation checks.
- Ensured that all changes align with performance goals, maintaining O(1) complexity for access patterns while enhancing usability and safety.

Author: TKorr