move Windows code into a module #1017

Job	Run time
Test CUDA support	25s
MSRV (ubuntu-latest)	20s
Clippy	7s
MSRV (windows-latest)	39s
Test aarch64-apple-tvos	3m 44s
Rustfmt	6s
Test (stable)	31s
Test (beta)	34s
Test (nightly)	33s
Test (linux32)	44s
Test (macos)	1m 23s
Test (aarch64-ios)	1m 47s
Test (win32)	1m 35s
Test (win64)	1m 28s
Test (mingw32)	1m 32s
Test (mingw64)	1m 55s
Test (windows-2019)	1m 59s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 48s
	22m 10s

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