chacha20: add `*Rng` equivalence proptests with `rand_core` #709

Job	Run time
build (1.85.0, thumbv7em-none-eabi)	24s
build (1.85.0, wasm32-unknown-unknown)	19s
test (stable)	22s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	22s
test (1.85.0)	20s
minimal-versions / minimal-versions	31s
	2m 41s

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