[Rust] Refactor union case patterns #1582

Job	Run time
build-dart	1m 55s
build-integration	3m 3s
build-javascript (ubuntu-latest)	4m 4s
analyzers	6m 2s
build-javascript (windows-latest)	6m 4s
build-python (ubuntu-latest, 3.10)	2m 54s
build-rust (default)	3m 16s
build-standalone	5m 6s
build-python (ubuntu-latest, 3.11)	2m 38s
build-rust (no_std)	3m 17s
build-typescript	2m 25s
build-python (ubuntu-latest, 3.12)	2m 41s
build-rust (threaded)	3m 14s
build-python (windows-latest, 3.10)	4m 26s
Check that solution can be restored	20s
verify-linting	27s
build-python (windows-latest, 3.11)	4m 19s
build-python (windows-latest, 3.12)	4m 17s
	1h 0m 28s

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