fix: make Debug impl for unions opaque #367

Job	Run time
Check #[cfg]s	10s
Style check	19s
Test tier1 (i686-unknown-linux-gnu, true, ubuntu-24.04)	3m 42s
Verify build (stable, ubuntu-24.04)	10s
Test tier1 (x86_64-unknown-linux-gnu, true, ubuntu-24.04)	3m 17s
Verify build (stable, macos-14)	17s
Verify build (stable, windows-2022)	8s
Test tier1 (aarch64-apple-darwin, macos-14)	1m 0s
Verify build (nightly, ubuntu-24.04)	13s
Test tier1 (x86_64-pc-windows-gnu, windows-2022, 64, x86_64)	3m 8s
Verify build (nightly, macos-14)	25s
Test tier1 (x86_64-pc-windows-msvc, windows-2022)	2m 57s
Verify build (nightly, windows-2022)	16s
Test tier1 (i686-pc-windows-msvc, windows-2022)	2m 57s
Verify build (1.63.0, ubuntu-24.04)	24s
Verify build (1.63.0, macos-14)	25s
Verify build (1.63.0, windows-2022)	18s
Verify build (beta, ubuntu-24.04)	12s
Test tier2	0s
Test tier2 VM	0s
success	1s
	20m 19s

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