zeroize: add unsafe `zeroize_flat_type` function (#1045) #1605

Job	Run time
build (1.41.0, thumbv7em-none-eabi)	11s
test (1.41.0)	10s
build (1.41.0, wasm32-unknown-unknown)	13s
test (stable)	23s
build (stable, thumbv7em-none-eabi)	12s
build (stable, wasm32-unknown-unknown)	26s
minimal-versions / minimal-versions	13s
	1m 48s

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