* Rework `needle_map.CompactMap()` to maximize memory efficiency.
* Use a memory-efficient structure for `CompactMap` needle value entries.
This slightly complicates the code, but makes a **massive** difference
in memory efficiency - preliminary results show a ~30% reduction in
heap usage, with no measurable performance impact otherwise.
* Clean up type for `CompactMap` chunk IDs.
* Add a small comment description for `CompactMap()`.
* Add the old version of `CompactMap()` for comparison purposes.
