Uses IntSet for dead slots

[brooksprumo]

Problem

When purging dead accounts, we also track dead slots. That is done by keeping a list, currently as a HashSet, of slots. This list could even be a vector, as we don't need the cryptographic security of the default HashSet's hashing function. Since we do lookups, a HashSet is nice. We can use IntSet to keep the nice lookups and remove the slow hashing, as the slot itself if sufficiently unique.

Summary of Changes

Use IntSet for dead slots

[codecov]

Codecov Report

Merging #33529 (b738e63) into master (f714a44) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master   #33529   +/-   ##
=======================================
  Coverage    81.7%    81.7%           
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  Files         805      805           
  Lines      218111   218111           
=======================================
+ Hits       178360   178367    +7     
+ Misses      39751    39744    -7     

[HaoranYi]

lgtm