replace pyrsistent.PMap, immutables.Map, immutabledict with constantdict (#884)

* replace pyrsistent.PMap, immutables.Map with immutabledict

* go back to immutables.Map for Tree

* ruff fixes

* spelling

* switch to constantdict

* lint fixes

* doc fix

* work around doc failure

* remove some spurious changes

* clean up types a bit

* simplify with_kernel