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digest-io: initial implementation #737

Job	Run time
test (stable)	16s
test (1.85.0)	16s
build (1.85.0, thumbv7em-none-eabi)	17s
build (stable, wasm32-unknown-unknown)	22s
build (stable, thumbv7em-none-eabi)	20s
build (1.85.0, wasm32-unknown-unknown)	19s
minimal-versions / minimal-versions	30s
	2m 20s

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