kem

Return `Result` from all `generate_*` methods (#1379) #778

Job	Run time
build (1.61.0, thumbv7em-none-eabi)	6m 12s
test-msrv	3m 31s
test (1.61.0)	6m 15s
test (stable)	37s
build (1.61.0, wasm32-unknown-unknown)	6m 8s
build (stable, thumbv7em-none-eabi)	9s
build (stable, wasm32-unknown-unknown)	12s
	23m 4s

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