Optimize exception value storage #1257

Job	Run time
abi_cafe (macos-13, x86_64-apple-darwin)	36s
abi_cafe (macos-latest, aarch64-apple-darwin)	24s
abi_cafe (ubuntu-24.04-arm, aarch64-unknown-linux-gnu)	29s
abi_cafe (ubuntu-latest, x86_64-unknown-linux-gnu)	21s
abi_cafe (windows-latest, x86_64-pc-windows-msvc)	30s
	2m 20s

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