Add `unique` function. #984

Job	Run time
Flake8	23s
Tests for downstream project loopy	5m 0s
Mypy	37s
Documentation	35s
Tests for downstream project pytato	9m 32s
Pylint	41s
Pytest on Py3.8	27s
Pytest without Numpy	29s
Validate CITATION.cff	9s
Pytest on Py3.9	28s
Pytest on Py3.x	33s
Pytest on Pypypy3.8	40s
	19m 34s

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