chacha20: add `*Rng` equivalence proptests with `rand_core` #686

Job	Run time
test (1.85.0)	23s
test (stable)	23s
build (stable, wasm32-unknown-unknown)	25s
build (1.85.0, thumbv7em-none-eabi)	23s
build (1.85.0, wasm32-unknown-unknown)	22s
build (stable, thumbv7em-none-eabi)	23s
minimal-versions / minimal-versions	35s
	2m 54s

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