chacha20: apply `unreachable_pub` lint (#570) #941

Job	Run time
test (1.85.0)	24s
build (1.85.0, thumbv7em-none-eabi)	19s
build (stable, wasm32-unknown-unknown)	18s
test (stable)	12s
minimal-versions / minimal-versions	27s
build (1.85.0, wasm32-unknown-unknown)	22s
build (stable, thumbv7em-none-eabi)	18s
	2m 20s

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