XXH128State: return self to enable chaining #4057

Job	Run time
Linux -> Pre-checks	17s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 2s
Linux -> Clang-x64, Debug	13m 39s
Linux -> GCC-x64, Release	11m 3s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 29s
Linux -> GCC-x64, Debug	12m 33s
Linux -> Clang-x64, Release	10m 59s
	1h 3m 2s

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