SHA3 xof shake128 and shake256 #35

Job	Run time
doctests	1m 2s
test (ubuntu-latest, x64, 1.6)	1m 20s
test (ubuntu-latest, x64, nightly)	4m 56s
test (ubuntu-latest, x86, 1.6)	1m 15s
test (ubuntu-latest, x86, nightly)	5m 16s
test (macOS-latest, x64, 1.6)	3m 6s
test (macOS-latest, x64, nightly)	4m 33s
test (windows-latest, x64, 1.6)	10m 7s
test (windows-latest, x64, nightly)	9m 57s
test (windows-latest, x86, 1.6)	10m 36s
test (windows-latest, x86, nightly)	10m 21s
	1h 2m 29s

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