* Sparse vector index
* better name
* fill properly postings
* conflict
* easy reviews
* use unstable sort
* use optional instead of bool flag
* clearer score type and future direction
* NonZeroUsize length for FixedLengthPriorityQueue
* rename id to record_id
* cleanup dimension inverted index
* standard Iterator
* code review
* Shift comment
* early return if no matching posting lists
* sprinkle iterators
* privatize the means of production
---------
Co-authored-by: timvisee <tim@visee.me>
