SHA3 xof shake128 and shake256 #36

Job	Run time
doctests	1m 32s
test (ubuntu-latest, x64, 1.6)	1m 49s
test (ubuntu-latest, x64, nightly)	3m 48s
test (ubuntu-latest, x86, 1.6)	1m 20s
test (ubuntu-latest, x86, nightly)	3m 50s
test (macOS-latest, x64, 1.6)	2m 31s
test (macOS-latest, x64, nightly)	5m 0s
test (windows-latest, x64, 1.6)	10m 14s
test (windows-latest, x64, nightly)	11m 33s
test (windows-latest, x86, 1.6)	12m 32s
test (windows-latest, x86, nightly)	15m 38s
	1h 9m 47s

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