bessels revisited #276

Job	Run time
macosx (-O0)	25m 24s
linux (g++, -O3 -DNDEBUG)	10m 26s
macosx (-O0 -stdlib=libc++, -stdlib=libc++)	24m 25s
macosx (-O3 -flto -DNDEBUG)	14m 23s
linux (clang++, -O3 -DNDEBUG)	16m 8s
linux (g++, -O3 -DNDEBUG -mavx2)	9m 2s
macosx (-O3 -flto -DNDEBUG -stdlib=libc++, -stdlib=libc++)	18m 27s
linux (clang++, -O3 -DNDEBUG -mavx2)	14m 18s
linux (g++, -O3 -DNDEBUG -march=skylake-avx512)	10m 49s
linux (clang++, -O3 -DNDEBUG -march=skylake-avx512)	13m 14s
	2h 36m 36s

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