remove coupling of ShrinkCandidates to HashMap

[jeffwashington]

Problem

Working on improving startup time.
ShrinkCandidates is a type. The type is currently HashMap. Many places in code use
let v = ShrinkCandidates: HashMap::new()

Summary of Changes

replace with:
let v = ShrinkCandidates::new()
to decouple the types.

Fixes #

[codecov]

Codecov Report

Merging #33176 (4f2df57) into master (528a03f) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           master   #33176     +/-   ##
=========================================
- Coverage    82.1%    82.1%   -0.1%     
=========================================
  Files         785      785             
  Lines      211205   211205             
=========================================
- Hits       173444   173426     -18     
- Misses      37761    37779     +18