kem

Return `Result` from all `generate_*` methods #773

Job	Run time
build (1.61.0, thumbv7em-none-eabi)	5m 22s
build (1.61.0, wasm32-unknown-unknown)	6m 44s
test (1.61.0)	4m 9s
test-msrv	4m 59s
build (stable, thumbv7em-none-eabi)	15s
test (stable)	36s
build (stable, wasm32-unknown-unknown)	16s
	22m 21s

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