Fix constant handling in `PersistentHashWalkMapper` #619

Job	Run time
Documentation	39s
Flake8	16s
Pylint	53s
Tests for downstream project loopy	4m 4s
Tests for downstream project pytential	49m 56s
Pytest on Py3.8	26s
Tests for downstream project pytato	5m 42s
Pytest on Py3.9	27s
Pytest on Py3.x	37s
	1h 3m 0s

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