Windows build system support

[emmatyping]

To build on Windows, the _zstd module needs Meta's libzstd to be added to cpython-source-deps. I have a branch with that set up, but I need to update the MSVC project files to build libzstd.

ref https://devguide.python.org/developer-workflow/extension-modules/#updating-msvc-project-files

[emmatyping]

With 2fbcae0, this is now implemented.

[emmatyping]

Oh also, I verified that the test_zstd, test_zipfile, and test_tarfile tests pass on 32 and 64 bit Windows.