zeroize: move `zeroize_derive` to toplevel #1654

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	48s
build (1.65.0, wasm32-unknown-unknown)	46s
test (1.65.0)	50s
build (stable, thumbv7em-none-eabi)	18s
test (stable)	15s
build (stable, wasm32-unknown-unknown)	13s
	3m 10s

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