Adding a zeroize function for flat types #589

Job	Run time
test (i686-unknown-linux-gnu, ubuntu-latest, 1.60.0, sudo apt update && sudo apt install gcc-mult...	2m 40s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	25s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.60.0)	2m 30s
aarch64 (aarch64-unknown-linux-gnu, 1.60.0)	2m 54s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	18s
aarch64 (aarch64-unknown-linux-gnu, stable)	43s
test (x86_64-apple-darwin, macos-latest, 1.60.0)	3m 50s
build (1.60.0, armv7a-none-eabi)	2m 25s
test (x86_64-apple-darwin, macos-latest, stable)	39s
build (1.60.0, thumbv7em-none-eabi)	2m 23s
cross (powerpc-unknown-linux-gnu, 1.60.0)	2m 50s
test (x86_64-pc-windows-msvc, windows-latest, 1.60.0)	3m 11s
build (1.60.0, wasm32-unknown-unknown)	2m 24s
cross (powerpc-unknown-linux-gnu, stable)	34s
test (x86_64-pc-windows-msvc, windows-latest, stable)	56s
build (stable, armv7a-none-eabi)	7s
build (stable, thumbv7em-none-eabi)	13s
build (stable, wasm32-unknown-unknown)	17s
minimal-versions / minimal-versions	42s
	30m 1s

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