Uses IntSet for uncleaned roots during index generation

[brooksprumo]

Problem

Index generation identifies uncleaned rooted slots. It puts them in a HashSet. The slot itself is already a unique identifier, and furthermore, there won't be any collisions here. We also don't need cryptographic security for the HashSet. Instead, we can use an IntSet to skip hashing the slot.

Summary of Changes

Replace HashSet with IntSet.

[codecov]

Codecov Report

Merging #34008 (fc22361) into master (bba6ea2) will decrease coverage by 0.1%.
Report is 4 commits behind head on master.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           master   #34008     +/-   ##
=========================================
- Coverage    81.9%    81.9%   -0.1%     
=========================================
  Files         811      811             
  Lines      219347   219347             
=========================================
- Hits       179732   179709     -23     
- Misses      39615    39638     +23     

[jeffwashington]

lgtm