fix: make Debug impl for unions opaque #371

Job	Run time
Test tier1 (i686-unknown-linux-gnu, true, ubuntu-24.04)	3m 39s
Verify build (stable, ubuntu-24.04)	2m 30s
Test tier1 (x86_64-unknown-linux-gnu, true, ubuntu-24.04)	3m 26s
Verify build (stable, macos-14)	15s
Test tier1 (aarch64-apple-darwin, macos-14)	1m 0s
Verify build (stable, windows-2022)	2m 16s
Test tier1 (x86_64-pc-windows-gnu, windows-2022, 64, x86_64)	4m 41s
Verify build (nightly, ubuntu-24.04)	2m 46s
Test tier1 (x86_64-pc-windows-msvc, windows-2022)	3m 14s
Check #[cfg]s	12s
Verify build (nightly, macos-14)	53s
Test tier1 (i686-pc-windows-msvc, windows-2022)	4m 30s
Style check	13s
Verify build (nightly, windows-2022)	2m 39s
Verify build (1.63.0, ubuntu-24.04)	2m 47s
Verify build (1.63.0, macos-14)	54s
Verify build (1.63.0, windows-2022)	2m 39s
Verify build (beta, ubuntu-24.04)	2m 39s
Test tier2 VM	0s
Test tier2	0s
success	1s
	41m 14s

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