Refactor S3 FIFO eviction strategy and enhance iterator implementations

- Updated the eviction logic to prioritize Small queue eviction based on its capacity, improving cache efficiency.
- Introduced `small_capacity` method to expose the maximum entries in the Small queue, enhancing usability.
- Added `FusedIterator` and `Debug` implementations for `Iter`, `Keys`, `Values`, and `IntoIter`, improving iterator functionality and debugging support.
- Enhanced documentation for eviction strategy and iterator methods, providing clearer guidance for users.

Author: TKorr