implement `KeyInit` for `aes-kw::Kek` and `BeltKwp` #16

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	15s
build (1.81.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	10s
test (1.81.0)	14s
build (stable, wasm32-unknown-unknown)	9s
test (stable)	19s
minimal-versions / minimal-versions	15s
	1m 41s

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