zeroize: move `zeroize_derive` to toplevel #1644

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	51s
build (1.65.0, wasm32-unknown-unknown)	47s
test (1.65.0)	47s
build (stable, thumbv7em-none-eabi)	12s
test (stable)	9s
build (stable, wasm32-unknown-unknown)	13s
minimal-versions / minimal-versions	19s
	3m 18s

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