Switch immutables.Map and pyrsistent.PMap to constantdict #3082

Job	Run time
Flake8	22s
Mypy	2m 4s
Tests for downstream project Firedrake	15m 19s
Documentation	2m 5s
Tests for downstream project arraycontext	2m 52s
Conda Examples	2m 24s
Conda Pytest (ubuntu-latest)	4m 21s
Tests for downstream project meshmode	10m 39s
Pylint	2m 16s
Conda Pytest (macos-latest)	6m 31s
Tests for downstream project grudge	9m 29s
Conda Pytest with Intel CL	4m 25s
Tests for downstream project pytential	33m 51s
Conda Pytest without arg check	4m 9s
Conda Pytest Twice (for cache behavior)	5m 36s
Validate CITATION.cff	8s
Tests for downstream project pytato	5m 34s
	1h 52m 5s

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