Add Dstack converter

[disktnk]

t/o

[codecov-io]

Codecov Report

Merging #168 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
+ Coverage   88.51%   88.62%   +0.11%     
==========================================
  Files          24       24              
  Lines        1332     1345      +13     
==========================================
+ Hits         1179     1192      +13     
  Misses        153      153

Impacted Files	Coverage Δ
onnx_chainer/mapping.py	90% <ø> (ø)	⬆️
onnx_chainer/functions/__init__.py	100% <100%> (ø)	⬆️
onnx_chainer/functions/array.py	89.27% <100%> (+0.58%)	⬆️

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[disktnk]

ChainerCI, test this please

[pfn-ci-bot]

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