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