implement `KeyInit` for `aes-kw::Kek` and `BeltKwp` #76

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	15s
test (1.81.0)	20s
test (stable)	18s
build (1.81.0, wasm32-unknown-unknown)	21s
build (stable, thumbv7em-none-eabi)	10s
build (stable, wasm32-unknown-unknown)	10s
minimal-versions / minimal-versions	23s
	1m 57s

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