der: add `SequenceRef::as_bytes` and `AsRef<[u8]>` impl #1223

Job	Run time
minimal-versions	0s
build (1.75.0, thumbv7em-none-eabi)	29s
build (1.75.0, wasm32-unknown-unknown)	28s
build (stable, thumbv7em-none-eabi)	37s
test (1.75.0)	1m 45s
build (stable, wasm32-unknown-unknown)	34s
test (stable)	1m 47s
	5m 40s

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