ml-kem: factor `DecapsulationKey`/`EncapsulationKey` into modules (#225) #518

Job	Run time
set-msrv / set-msrv	5s
minimal-versions	0s
test (1.85.0)	1m 12s
cross (powerpc-unknown-linux-gnu, stable)	1m 35s
test (stable)	1m 9s
cross (powerpc-unknown-linux-gnu, 1.85.0)	1m 26s
no_std (stable, thumbv7em-none-eabi)	23s
no_std (1.85.0, thumbv7em-none-eabi)	29s
no_std (1.85.0, wasm32-unknown-unknown)	40s
no_std (stable, wasm32-unknown-unknown)	19s
	7m 18s

Provide feedback