zeroize: move `zeroize_derive` to toplevel #637

Job	Run time
build (1.60.0, armv7a-none-eabi)	48s
cross (aarch64-unknown-linux-gnu, 1.60.0)	1m 13s
build (1.60.0, thumbv7em-none-eabi)	46s
cross (aarch64-unknown-linux-gnu, stable)	39s
build (1.60.0, wasm32-unknown-unknown)	47s
cross (powerpc-unknown-linux-gnu, 1.60.0)	1m 15s
build (stable, armv7a-none-eabi)	8s
cross (powerpc-unknown-linux-gnu, stable)	54s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.60.0, sudo apt update && sudo apt install gcc-mult...	52s
build (stable, thumbv7em-none-eabi)	8s
x86 (x86_64-unknown-linux-gnu, 1.72.0)	22s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	34s
build (stable, wasm32-unknown-unknown)	18s
x86 (x86_64-unknown-linux-gnu, stable)	17s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.60.0)	48s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	16s
test (x86_64-apple-darwin, macos-latest, 1.60.0)	49s
test (x86_64-apple-darwin, macos-latest, stable)	17s
test (x86_64-pc-windows-msvc, windows-latest, 1.60.0)	1m 9s
test (x86_64-pc-windows-msvc, windows-latest, stable)	43s
minimal-versions / minimal-versions	51s
	13m 54s

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