split miscellaneous code into a new command_helpers module #999

Job	Run time
MSRV (ubuntu-latest)	20s
Test (stable)	16s
MSRV (windows-latest)	37s
Test (beta)	15s
Test CUDA support	23s
Test aarch64-apple-tvos	59s
Test (nightly)	13s
Rustfmt	7s
Test (linux32)	18s
Clippy	32s
Test (macos)	24s
Test (aarch64-ios)	5s
Test (win32)	17s
Test (win64)	18s
Test (mingw32)	22s
Test (mingw64)	22s
Test (windows-2019)	21s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	18s
	6m 27s

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