Implement `Unique::fold`

[kinto-b]

Relates to #755

$ cargo bench --bench specializations unique/fold

unique/fold             time:   [11.672 µs 11.710 µs 11.753 µs]
                        change: [-5.4377% -4.4270% -3.6741%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  4 (4.00%) high mild
  3 (3.00%) high severe

[codecov[bot]]

Codecov Report

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

Project coverage is 94.49%. Comparing base (6814180) to head (db0c386). Report is 54 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #919      +/-   ##
==========================================
+ Coverage   94.38%   94.49%   +0.10%     
==========================================
  Files          48       48              
  Lines        6665     6884     +219     
==========================================
+ Hits         6291     6505     +214     
- Misses        374      379       +5     

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