Adding a zeroize function for flat types #582

Job	Run time
build (1.60.0, armv7a-none-eabi)	2m 18s
build (1.60.0, thumbv7em-none-eabi)	2m 14s
cross (powerpc-unknown-linux-gnu, 1.60.0)	2m 41s
aarch64 (aarch64-unknown-linux-gnu, 1.60.0)	2m 43s
build (1.60.0, wasm32-unknown-unknown)	2m 11s
cross (powerpc-unknown-linux-gnu, stable)	32s
aarch64 (aarch64-unknown-linux-gnu, stable)	43s
build (stable, armv7a-none-eabi)	8s
build (stable, thumbv7em-none-eabi)	7s
build (stable, wasm32-unknown-unknown)	8s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.60.0, sudo apt update && sudo apt install gcc-mult...	2m 39s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	51s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.60.0)	2m 23s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	33s
test (x86_64-apple-darwin, macos-latest, 1.60.0)	4m 3s
test (x86_64-apple-darwin, macos-latest, stable)	44s
test (x86_64-pc-windows-msvc, windows-latest, 1.60.0)	3m 4s
test (x86_64-pc-windows-msvc, windows-latest, stable)	1m 4s
minimal-versions / minimal-versions	40s
	29m 46s

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