kem

Return `Result` from all `generate_*` methods #768

Job	Run time
build (1.61.0, thumbv7em-none-eabi)	3m 31s
test (1.61.0)	4m 0s
test-msrv	5m 19s
build (1.61.0, wasm32-unknown-unknown)	3m 41s
test (stable)	47s
build (stable, thumbv7em-none-eabi)	11s
build (stable, wasm32-unknown-unknown)	24s
	17m 53s

Provide feedback