SHA3 xof shake128 and shake256 #34

Job	Run time
doctests	56s
test (ubuntu-latest, x64, 1.6)	1m 18s
test (ubuntu-latest, x64, nightly)	3m 47s
test (ubuntu-latest, x86, 1.6)	2m 28s
test (ubuntu-latest, x86, nightly)	2m 46s
test (macOS-latest, x64, 1.6)	2m 30s
test (macOS-latest, x64, nightly)	5m 2s
test (windows-latest, x64, 1.6)	12m 7s
test (windows-latest, x64, nightly)	14m 59s
test (windows-latest, x86, 1.6)	8m 48s
test (windows-latest, x86, nightly)	9m 16s
	1h 3m 57s

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