zeroize: move `zeroize_derive` to toplevel #551

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	43s
build (1.65.0, wasm32-unknown-unknown)	50s
build (stable, thumbv7em-none-eabi)	23s
test (1.65.0)	47s
test (stable)	11s
build (stable, wasm32-unknown-unknown)	9s
minimal-versions / minimal-versions	17s
	3m 20s

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