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} >>>4inaFalse>>>

Got

>>>a={1,2,3} >>>b={4,5,6} >>>a|b<setinstanceat0xc0003f8000>>>>a<setinstanceat0xc0003f8000>>>>4inaTrue>>>

[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