move Windows code into a module #982

Job	Run time
Test aarch64-apple-tvos	3m 46s
Test (stable)	33s
Clippy	31s
Test (beta)	26s
Test CUDA support	23s
MSRV (ubuntu-latest)	20s
Test (nightly)	26s
MSRV (windows-latest)	37s
Test (linux32)	34s
Rustfmt	8s
Test (macos)	1m 19s
Test (aarch64-ios)	35s
Test (win32)	1m 2s
Test (win64)	2m 30s
Test (mingw32)	1m 31s
Test (mingw64)	1m 29s
Test (windows-2019)	1m 43s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 11s
	20m 4s

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