der: add `SequenceRef::as_bytes` and `AsRef<[u8]>` impl #653

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	26s
minimal-versions	0s
test (1.75.0)	33s
build (1.75.0, wasm32-unknown-unknown)	30s
test (stable)	32s
build (stable, thumbv7em-none-eabi)	28s
build (stable, wasm32-unknown-unknown)	30s
	2m 59s

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