test hashtable expansion/rebuild.
hashindex_lookup:
- return -2 for a compact / completely full hashtable
- return -1 and (via start_idx pointer) the deleted/tombstone bucket index.
fix size assertion (we add 1 element to trigger rebuild)
fix upper_limit check - since we'll be adding 1 to num_entries below,
the condition should be >=:
hashindex_compact: set min_empty/upper_limit
Co-authored-by: Dan Christensen <jdc+github@uwo.ca>