ml-kem: impl `KeyExport` for `DecapsulationKey` #418

Job	Run time
set-msrv / set-msrv	6s
minimal-versions	0s
test (1.85.0)	36s
test (stable)	32s
cross (powerpc-unknown-linux-gnu, stable)	48s
no_std (stable, wasm32-unknown-unknown)	14s
no_std (stable, thumbv7em-none-eabi)	29s
no_std (1.85.0, wasm32-unknown-unknown)	22s
cross (powerpc-unknown-linux-gnu, 1.85.0)	51s
no_std (1.85.0, thumbv7em-none-eabi)	26s
	4m 24s

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