digest: return `CtOutput::into_bytes` method #2258

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	20s
derive (1.71.0)	16s
build (1.71.0, wasm32-unknown-unknown)	20s
derive (stable)	8s
doc	24s
build (stable, thumbv7em-none-eabi)	27s
build (stable, wasm32-unknown-unknown)	17s
test (1.71.0)	21s
test (stable)	21s
minimal-versions / minimal-versions	20s
	3m 14s

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