kem

Return `Result` from all `generate_*` methods #767

Job	Run time
build (1.61.0, thumbv7em-none-eabi)	3m 41s
build (1.61.0, wasm32-unknown-unknown)	3m 38s
build (stable, thumbv7em-none-eabi)	20s
test (1.61.0)	4m 8s
test-msrv	6m 10s
build (stable, wasm32-unknown-unknown)	8s
test (stable)	46s
	18m 51s

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