Implement new `kem::Kem` trait #515

Job	Run time
set-msrv / set-msrv	7s
minimal-versions	0s
test (stable)	1m 9s
no_std (1.85.0, thumbv7em-none-eabi)	26s
no_std (stable, thumbv7em-none-eabi)	24s
cross (powerpc-unknown-linux-gnu, 1.85.0)	1m 32s
no_std (stable, wasm32-unknown-unknown)	20s
test (1.85.0)	1m 11s
no_std (1.85.0, wasm32-unknown-unknown)	29s
cross (powerpc-unknown-linux-gnu, stable)	1m 24s
	7m 2s

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