Allow isless() on unordered CategoricalValues

[nalimilan]

< still raises an error for unordered values to protect from meaningless
comparisons, but isless() is now allowed to support sort() on unordered arrays,
where this operation is useful. This behavior is consistent with e.g. NaN being
sorted last by isless() despite the comparison having no mathematical signification.

Cc: @cjprybol

[cjprybol]

Same here, thanks for writing these up!

[coveralls]

Coverage decreased (-0.1%) to 90.744% when pulling 0a1cb4c on nl/isless into 388575a on master.

[codecov-io]

Codecov Report

Merging #58 into master will increase coverage by 0.65%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   90.89%   91.54%   +0.65%     
==========================================
  Files           9        9              
  Lines         494      532      +38     
==========================================
+ Hits          449      487      +38     
  Misses         45       45

Impacted Files	Coverage Δ
src/value.jl	74.5% <75%> (+13.9%)	✅
src/array.jl	93.06% <ø> (+0.61%)	✅

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[coveralls]

Coverage increased (+0.4%) to 91.304% when pulling 9e59866 on nl/isless into 388575a on master.