Keep secret string conversion panic-free #206

Job	Run time
Format, lint, test, and audit	1m 15s
Miri	18m 16s
Platform tests (macos-latest)	25s
Platform tests (macos-15)	28s
Platform tests (ubuntu-latest)	34s
no_std target build (x86_64-unknown-freebsd)	18s
Platform tests (windows-latest)	36s
Platform tests (macos-15-intel)	48s
Platform tests (macos-26)	24s
no_std target build (x86_64-unknown-linux-gnu)	17s
no_std target build (thumbv7em-none-eabihf)	18s
no_std target build (aarch64-unknown-linux-gnu)	18s
no_std target build (wasm32-unknown-unknown)	18s
	24m 15s

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