zeroize: move `zeroize_derive` to toplevel #1139

Job	Run time
macos (stable)	14s
linux (i686-unknown-linux-gnu, 1.40.0, sudo apt update && sudo apt install gcc-multilib)	1m 1s
windows (x86_64-pc-windows-gnu, stable)	45s
linux (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	25s
linux (x86_64-unknown-linux-gnu, 1.40.0)	42s
linux (x86_64-unknown-linux-gnu, stable)	7s
build-only	9s
cross (aarch64-unknown-linux-gnu, stable)	34s
minimal-versions / minimal-versions	16s
	4m 13s

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