Adds iter_ones() to RollingBitField

[brooksprumo]

Problem

During shrink, we are given a set of 'candidate slots' to inspect. Currently, these candidates are a HashSet (with the nohash-hasher). To make this faster, we can change the ShrinkCandidates to a RollingBitField. However, RollingBitField does not have a method to iterate over the set bits, so this means we would have to create a Vec of all the set bits, even though we don't need the Vec.

It would be better to avoid this unnecessary allocation, and instead just iterate the set bits.

Summary of Changes

Add iter_ones() to RollingBitField.

[codecov]

Codecov Report

Merging #33956 (186fd44) into master (a4a6602) will increase coverage by 0.0%.
Report is 8 commits behind head on master.
The diff coverage is 91.8%.

@@           Coverage Diff           @@
##           master   #33956   +/-   ##
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  Coverage    81.9%    81.9%           
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  Files         809      810    +1     
  Lines      218447   218484   +37     
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+ Hits       178942   179022   +80     
+ Misses      39505    39462   -43     

[HaoranYi]

lgtm.