digest: add missing `Debug` impl for `HashReader` and `HashWriter` (#… #2749

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	34s
build (1.73.0, wasm32-unknown-unknown)	40s
build (stable, thumbv7em-none-eabi)	32s
minimal-versions	0s
build (stable, wasm32-unknown-unknown)	28s
test (1.73.0)	27s
test (stable)	26s
test (nightly)	27s
	3m 34s

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