zeroize: move `zeroize_derive` to toplevel #496

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	48s
cross (aarch64-unknown-linux-gnu, 1.60.0)	1m 11s
build (1.60.0, wasm32-unknown-unknown)	46s
cross (aarch64-unknown-linux-gnu, stable)	32s
cross (powerpc-unknown-linux-gnu, 1.60.0)	1m 9s
build (stable, thumbv7em-none-eabi)	7s
cross (powerpc-unknown-linux-gnu, stable)	40s
build (stable, wasm32-unknown-unknown)	17s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.60.0, sudo apt update && sudo apt install gcc-mult...	1m 0s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.60.0)	45s
test (x86_64-pc-windows-msvc, windows-latest, 1.60.0)	1m 5s
test (x86_64-apple-darwin, macos-latest, 1.60.0)	59s
minimal-versions / minimal-versions	28s
	9m 47s

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