Implement dup

[st0012]

The default dup method returns the receiver object. Mutable data structures like Object, Array, String and Hash have their own implementation to override the default behavior for real duplication.

[codecov]

Codecov Report

Merging #791 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
+ Coverage   80.22%   80.31%   +0.09%     
==========================================
  Files          54       54              
  Lines        7397     7431      +34     
==========================================
+ Hits         5934     5968      +34     
  Misses       1236     1236              
  Partials      227      227

Impacted Files	Coverage Δ
vm/object.go	80.59% <100%> (+1.23%)	⬆️
vm/error.go	70.96% <100%> (ø)	⬆️
vm/array.go	96.17% <100%> (+0.05%)	⬆️
vm/class.go	85.8% <100%> (+0.17%)	⬆️
vm/string.go	98.27% <100%> (+0.01%)	⬆️
vm/hash.go	96.81% <100%> (+0.02%)	⬆️
vm/environment.go	100% <100%> (ø)	⬆️

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[64kramsystem]

👍

[hachi8833]

Thank you for the implementations!
I'll add the API docs to them.