bessels revisited #274

Job	Run time
macosx (-O0)	24m 18s
macosx (-O0 -stdlib=libc++, -stdlib=libc++)	24m 48s
linux (g++, -O3 -DNDEBUG)	11m 21s
macosx (-O3 -flto -DNDEBUG)	16m 48s
linux (clang++, -O3 -DNDEBUG)	15m 58s
macosx (-O3 -flto -DNDEBUG -stdlib=libc++, -stdlib=libc++)	19m 50s
linux (g++, -O3 -DNDEBUG -mavx2)	10m 10s
linux (clang++, -O3 -DNDEBUG -mavx2)	14m 57s
linux (g++, -O3 -DNDEBUG -march=skylake-avx512)	9m 53s
linux (clang++, -O3 -DNDEBUG -march=skylake-avx512)	12m 46s
	2h 40m 49s

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