Return `Result` from all `generate_*` methods #2094

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 42s
test (1.65.0)	3m 34s
build (1.65.0, wasm32-unknown-unknown)	3m 29s
test (stable)	21s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	1m 7s
	12m 36s

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