implement `KeyInit` for `aes-kw::Kek` and `BeltKwp` #78

Job	Run time
test (1.81.0)	23s
build (1.81.0, thumbv7em-none-eabi)	16s
test (stable)	14s
build (1.81.0, wasm32-unknown-unknown)	16s
build (stable, thumbv7em-none-eabi)	10s
build (stable, wasm32-unknown-unknown)	10s
minimal-versions / minimal-versions	24s
	1m 53s

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