Refine shadow implementation #182

Job	Run time
build (ubuntu-latest, gcc, g++, Debug)	3m 21s
build (macos-latest, clang, clang++, Release)	1m 7s
build (windows-latest, cl, msvc, Debug)	2m 37s
build (macos-latest, clang, clang++, Debug)	44s
build (ubuntu-latest, gcc, g++, Release)	3m 41s
build (windows-latest, cl, msvc, Release)	3m 5s
	14m 35s

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