refactor: remove repetitive folds in `Z3Context` #7051

Job	Run time
compiler-warnings	2m 44s
build-mdbook	7s
format-check	4m 1s
docker-tests	20m 36s
integration-tests (ubuntu-latest, oopsla19)	19m 5s
scaladoc-check	2m 55s
integration-tests (ubuntu-latest, arrays)	9m 4s
unit-tests (ubuntu-latest)	5m 35s
integration-tests (macos-latest, oopsla19)	13m 19s
unit-tests (macos-latest)	4m 23s
	1h 21m 49s

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