chacha20: minor improvements #698

Job	Run time
build (1.85.0, wasm32-unknown-unknown)	25s
build (1.85.0, thumbv7em-none-eabi)	24s
test (1.85.0)	21s
test (stable)	20s
build (stable, wasm32-unknown-unknown)	32s
build (stable, thumbv7em-none-eabi)	21s
minimal-versions	0s
	2m 23s

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