fix[tool]: fix cross-compilation issues, add windows CI

[charles-cooper]

What I did

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Commit message

this commit fixes some `InputBundle` bugs on windows due to how
path-manipulation operations are implemented. it also fixes compilation
of cross-OS archives by settling on using posix paths as standard (this
follows the ZIP archive spec, and is explicit in the python stdlib
implementation of zipfile).

this commit also enables a windows job in the CI for tests, to prevent
bugs like these in the future and increase cross-OS coverage.

references:
- https://stackoverflow.com/a/8177003
- https://hg.python.org/cpython/file/2.7/Lib/zipfile.py#l295
- https://hg.python.org/cpython/file/2.7/Lib/zipfile.py#l1046

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 8.33333% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 50.29%. Comparing base (c276c94) to head (1afe87b).

:exclamation: Current head 1afe87b differs from pull request most recent head a43d2e8. Consider uploading reports for the commit a43d2e8 to get more accurate results

Files	Patch %	Lines
vyper/compiler/output_bundle.py	0.00%	7 Missing :warning:
vyper/cli/vyper_json.py	0.00%	2 Missing :warning:
vyper/compiler/input_bundle.py	33.33%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4014 +/- ## =========================================== - Coverage 90.95% 50.29% -40.67% =========================================== Files 106 106 Lines 15280 15282 +2 Branches 3365 3365 =========================================== - Hits 13898 7686 -6212 - Misses 946 6965 +6019 - Partials 436 631 +195 ```

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