set: Implement __or__ of set

[DoDaek]

Implement or of set
ISSUE: #76

[corona10]

Implementation is wrong

Expected

>>> a = {1,2,3}
>>> b = {4,5,6}
>>> a | b
{1, 2, 3, 4, 5, 6}
>>> 4 in a
False
>>>

Got

>>> a = {1,2,3}
>>> b = {4,5,6}
>>> a | b
<set instance at 0xc0003f8000>
>>> a
<set instance at 0xc0003f8000>
>>> 4 in a
True
>>>

[codecov-io]

Codecov Report

Merging #84 into master will increase coverage by 0.01%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   68.83%   68.84%   +0.01%     
==========================================
  Files          59       59              
  Lines       10534    10545      +11     
==========================================
+ Hits         7251     7260       +9     
- Misses       2774     2775       +1     
- Partials      509      510       +1

Impacted Files	Coverage Δ
py/set.go	37.64% <81.81%> (+6.56%)	⬆️

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[corona10]

LGTM