zeroize: move `zeroize_derive` to toplevel #1657

Job	Run time
build (1.57.0, thumbv7em-none-eabi)	13s
test (1.57.0)	13s
build (1.57.0, wasm32-unknown-unknown)	13s
test (stable)	6s
build (stable, thumbv7em-none-eabi)	15s
build (stable, wasm32-unknown-unknown)	7s
minimal-versions / minimal-versions	34s
	1m 41s

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