Fix union field access by representing unions as structs #415

Job	Run time
Test (ubuntu-24.04, aarch64-linux-android, x86_64-unknown-linux-gnu)	2m 17s
Lint	2m 18s
Test (macOS-latest, x86_64-apple-darwin)	2m 21s
Test (windows-2022, x86_64-pc-windows-msvc)	2m 2s
Test (ubuntu-24.04, x86_64-unknown-linux-gnu)	2m 18s
cargo-deny	1m 21s
test_success	1s
	12m 38s

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