Add subtle companion crate #241

Job	Run time
Format, lint, test, and audit	1m 23s
Miri	17m 58s
Platform tests (macos-latest)	39s
Platform tests (ubuntu-latest)	30s
no_std target build (x86_64-unknown-linux-gnu)	19s
no_std target build (x86_64-unknown-freebsd)	20s
Platform tests (macos-15-intel)	1m 7s
Platform tests (macos-15)	31s
Platform tests (macos-26)	31s
Platform tests (windows-latest)	49s
no_std target build (aarch64-unknown-linux-gnu)	22s
no_std target build (thumbv7em-none-eabihf)	19s
no_std target build (wasm32-unknown-unknown)	19s
	25m 7s

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