der: add `SequenceRef::as_bytes` and `AsRef<[u8]>` impl #1806

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	1m 0s
build (1.75.0, wasm32-unknown-unknown)	1m 0s
build (stable, thumbv7em-none-eabi)	59s
build (stable, wasm32-unknown-unknown)	57s
fuzz	2m 14s
minimal-versions	0s
test (1.75.0)	37m 41s
test (stable)	33m 47s
	1h 17m 38s

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