Implement `UniqueBy::fold`

[kinto-b]

Relates to #755

$ cargo bench --bench specializations unique_by/fold

unique_by/fold          time:   [13.702 µs 13.732 µs 13.766 µs]
                        change: [-6.8548% -6.5012% -6.1766%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.42%. Comparing base (6814180) to head (5a6fb6f). Report is 54 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #918      +/-   ##
==========================================
+ Coverage   94.38%   94.42%   +0.03%     
==========================================
  Files          48       48              
  Lines        6665     6887     +222     
==========================================
+ Hits         6291     6503     +212     
- Misses        374      384      +10     

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