der: conversions from `OctetString(Ref)` to `Vec`/`Bytes` #1547

Job	Run time
minimal-versions	0s
build (1.81.0, thumbv7em-none-eabi)	25s
build (1.81.0, wasm32-unknown-unknown)	29s
test (1.81.0)	1m 4s
build (stable, thumbv7em-none-eabi)	24s
test (stable)	1m 1s
build (stable, wasm32-unknown-unknown)	25s
	3m 48s

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