Pretty-printing for OneElement

[jishnub]

After this,

julia> B = OneElement(2, 1, 3);

julia> show(B)
OneElement(2, 1, 3)

The displayed form is a valid constructor, and is a compact representation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e9077c3) 99.89% compared to head (4d4173f) 99.89%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #348   +/-   ##
=======================================
  Coverage   99.89%   99.89%           
=======================================
  Files           8        8           
  Lines         934      938    +4     
=======================================
+ Hits          933      937    +4     
  Misses          1        1           

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