Add __new__ function and property of slice

[HyeockJinKim]

Issue #98

[codecov-io]

Codecov Report

Merging #99 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #99 +/- ##
==========================================
+ Coverage 69.09% 69.23% +0.14% 
==========================================
Files 60 60 Lines 10675 10692 +17 ==========================================
+ Hits 7376 7403 +27 + Misses 2781 2770 -11 - Partials 518 519 +1

Impacted Files	Coverage Δ
builtin/builtin.go	79.71% <100%> (+0.03%)	⬆️
py/slice.go	82% <100%> (+15.33%)	⬆️

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[HyeockJinKim]

@corona10
Can I get a review?

[corona10]

LGTM