Optimization: Vendor jobserver impl and rm thread spawning in parallel compile_objects #885

Job	Run time
Test CUDA support	35s
MSRV (ubuntu-latest)	29s
Test (stable)	32s
MSRV (windows-latest)	42s
Rustfmt	5s
Test (beta)	25s
Test (nightly)	26s
Test (linux32)	1m 2s
Test (macos)	55s
Test (aarch64-ios)	39s
Test (win32)	58s
Test (win64)	2m 7s
Test (mingw32)	1m 21s
Test (mingw64)	1m 16s
Test (windows-2019)	1m 22s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	57s
	13m 51s

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