Implement `unique()` S3 methods (#277)

[lewinfox]

Closes #277

• Implement unique.units()
• Implement unique.mixed_units()
• Add tests

[codecov[bot]]

Codecov Report

Merging #283 (00f23ac) into master (019b3f7) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
+ Coverage   94.41%   94.42%   +0.01%     
==========================================
  Files          18       18              
  Lines         877      879       +2     
==========================================
+ Hits          828      830       +2     
  Misses         49       49              

Impacted Files	Coverage Δ
R/misc.R	100.00% <100.00%> (ø)
R/mixed.R	98.14% <100.00%> (+0.03%)	:arrow_up:

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[Enchufa2]

LGTM, thanks!