der: add `SequenceRef::as_bytes` and `AsRef<[u8]>` impl #3107

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	49s
build (1.72.0, wasm32-unknown-unknown)	51s
build (stable, thumbv7em-none-eabi)	47s
minimal-versions	0s
build (stable, wasm32-unknown-unknown)	47s
test (1.72.0)	4m 36s
test (stable)	4m 35s
	12m 25s

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