Switch immutables.Map and pyrsistent.PMap to constantdict #3074

Job	Run time
Documentation	2m 3s
Conda Examples	1m 56s
Tests for downstream project Firedrake	15m 26s
Tests for downstream project arraycontext	2m 30s
Flake8	26s
Mypy	1m 50s
Pylint	2m 21s
Conda Pytest (ubuntu-latest)	2m 15s
Conda Pytest (macos-latest)	2m 31s
Conda Pytest with Intel CL	2m 35s
Tests for downstream project meshmode	4m 22s
Conda Pytest without arg check	2m 8s
Tests for downstream project grudge	3m 39s
Conda Pytest Twice (for cache behavior)	2m 24s
Tests for downstream project pytential	3m 32s
Validate CITATION.cff	8s
Tests for downstream project pytato	2m 39s
	52m 45s

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