SHA3 xof shake128 and shake256 #32

Job	Run time
doctests	1m 32s
test (ubuntu-latest, x64, 1.6)	1m 59s
test (ubuntu-latest, x64, nightly)	4m 23s
test (ubuntu-latest, x86, 1.6)	1m 14s
test (ubuntu-latest, x86, nightly)	2m 45s
test (macOS-latest, x64, 1.6)	3m 13s
test (macOS-latest, x64, nightly)	4m 21s
test (windows-latest, x64, 1.6)	11m 37s
test (windows-latest, x64, nightly)	15m 42s
test (windows-latest, x86, 1.6)	14m 10s
test (windows-latest, x86, nightly)	12m 23s
	1h 13m 19s

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