chacha20: adopt the new Generator API from rand_core #636

Job	Run time
test (1.85.0)	20s
test (stable)	21s
build (1.85.0, thumbv7em-none-eabi)	21s
build (stable, wasm32-unknown-unknown)	23s
build (1.85.0, wasm32-unknown-unknown)	21s
build (stable, thumbv7em-none-eabi)	16s
minimal-versions	0s
	2m 2s

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