move Windows code into a module #981

Job	Run time
Test aarch64-apple-tvos	3m 49s
Test CUDA support	29s
Clippy	4s
Test (stable)	29s
MSRV (ubuntu-latest)	20s
Rustfmt	7s
MSRV (windows-latest)	49s
Test (beta)	33s
Test (nightly)	42s
Test (linux32)	53s
Test (macos)	37s
Test (aarch64-ios)	46s
Test (win32)	41s
Test (win64)	43s
Test (mingw32)	41s
Test (mingw64)	37s
Test (windows-2019)	43s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	43s
	13m 46s

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