Optimization: Vendor jobserver impl and rm thread spawning in parallel compile_objects #862

Job	Run time
Test CUDA support	1m 11s
Rustfmt	7s
MSRV (ubuntu-latest)	4m 35s
Test (stable)	35s
MSRV (windows-latest)	5m 15s
Test (beta)	37s
Test (nightly)	35s
Test (linux32)	1m 11s
Test (macos)	1m 25s
Test (aarch64-ios)	1m 2s
Test (win32)	1m 18s
Test (win64)	2m 33s
Test (mingw32)	1m 42s
Test (mingw64)	1m 58s
Test (windows-2019)	1m 18s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	1m 31s
	26m 53s

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