chacha20: apply `unreachable_pub` lint (#570) #767

Job	Run time
build (stable, thumbv7em-none-eabi)	44s
test (stable)	22s
build (1.85.0, thumbv7em-none-eabi)	22s
build (1.85.0, wasm32-unknown-unknown)	19s
test (1.85.0)	22s
build (stable, wasm32-unknown-unknown)	18s
minimal-versions / minimal-versions	29s
	2m 56s

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