Adding a zeroize function for flat types #588

Job	Run time
cross (powerpc-unknown-linux-gnu, 1.60.0)	2m 42s
aarch64 (aarch64-unknown-linux-gnu, 1.60.0)	2m 54s
build (1.60.0, armv7a-none-eabi)	2m 27s
cross (powerpc-unknown-linux-gnu, stable)	27s
aarch64 (aarch64-unknown-linux-gnu, stable)	40s
build (1.60.0, thumbv7em-none-eabi)	2m 26s
build (1.60.0, wasm32-unknown-unknown)	2m 27s
build (stable, armv7a-none-eabi)	21s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.60.0, sudo apt update && sudo apt install gcc-mult...	26s
build (stable, thumbv7em-none-eabi)	8s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	24s
build (stable, wasm32-unknown-unknown)	13s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.60.0)	33s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	17s
test (x86_64-apple-darwin, macos-latest, 1.60.0)	26s
test (x86_64-apple-darwin, macos-latest, stable)	25s
test (x86_64-pc-windows-msvc, windows-latest, 1.60.0)	18s
test (x86_64-pc-windows-msvc, windows-latest, stable)	27s
minimal-versions / minimal-versions	21s
	18m 22s

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