Redo MSVC Compile-time constant error

[Thorius]

• Revert back to compile-time values for reduction types but replace const ReductionType declaration with static constexpr ReductionType.
• Add /permissive- flag when building on Windows to disable MSVC extensions and improve conformance.

[codecov-io]

Codecov Report

Merging #117 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #117   +/-   ##
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  Coverage     100%   100%           
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  Files           7      7           
  Lines          49     49           
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  Hits           49     49

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[Thorius]

I wanted to add /permissive- flag to MSVC builds to catch any standard conformance errors early, but the PyTorch headers appeared to cause issues with this on cl.exe 14.16 so I removed that change and only kept the static constexpr changes.

And example build: https://travis-ci.org/rusty1s/pytorch_scatter/jobs/650788515?utm_medium=notification&utm_source=github_status
Not sure what might cause this, but probably something like Unicode characters in the header or simply a bug in the compiler.

[rusty1s]

Thank you very much! This looks awesome.