Adding a zeroize function for flat types #585

Job	Run time
build (1.60.0, armv7a-none-eabi)	2m 15s
build (1.60.0, thumbv7em-none-eabi)	2m 19s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.60.0, sudo apt update && sudo apt install gcc-mult...	2m 41s
build (1.60.0, wasm32-unknown-unknown)	2m 13s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	30s
aarch64 (aarch64-unknown-linux-gnu, 1.60.0)	2m 49s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.60.0)	2m 21s
build (stable, armv7a-none-eabi)	10s
aarch64 (aarch64-unknown-linux-gnu, stable)	41s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	12s
build (stable, thumbv7em-none-eabi)	50s
test (x86_64-apple-darwin, macos-latest, 1.60.0)	3m 51s
build (stable, wasm32-unknown-unknown)	30s
test (x86_64-apple-darwin, macos-latest, stable)	28s
cross (powerpc-unknown-linux-gnu, 1.60.0)	2m 43s
cross (powerpc-unknown-linux-gnu, stable)	28s
test (x86_64-pc-windows-msvc, windows-latest, 1.60.0)	3m 13s
test (x86_64-pc-windows-msvc, windows-latest, stable)	1m 44s
minimal-versions / minimal-versions	39s
	30m 37s

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