cleaning + simpler arbo #287

Job	Run time
macosx (-O0)	26m 52s
linux (g++, -O3 -DNDEBUG)	23m 10s
macosx (-O0 -stdlib=libc++, -stdlib=libc++)	24m 20s
linux (clang++, -O3 -DNDEBUG)	33m 18s
macosx (-O3 -flto -DNDEBUG)	14m 5s
linux (g++, -O3 -DNDEBUG -mavx2)	15m 58s
macosx (-O3 -flto -DNDEBUG -stdlib=libc++, -stdlib=libc++)	16m 17s
linux (clang++, -O3 -DNDEBUG -mavx2)	21m 1s
linux (g++, -O3 -DNDEBUG -march=skylake-avx512)	19m 55s
linux (clang++, -O3 -DNDEBUG -march=skylake-avx512)	19m 20s
	3h 34m 16s

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