der: add `SequenceRef::as_bytes` and `AsRef<[u8]>` impl #3062

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	29s
minimal-versions	0s
build (1.72.0, wasm32-unknown-unknown)	32s
test (1.72.0)	1m 19s
build (stable, thumbv7em-none-eabi)	31s
test (stable)	1m 17s
build (stable, wasm32-unknown-unknown)	32s
	4m 40s

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