digest: return `CtOutput::into_bytes` method #2203

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 48s
test (1.65.0)	2m 10s
build (1.65.0, wasm32-unknown-unknown)	1m 51s
test (stable)	16s
build (stable, thumbv7em-none-eabi)	9s
build (stable, wasm32-unknown-unknown)	14s
	6m 28s

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