der: add `SequenceRef::as_bytes` and `AsRef<[u8]>` impl #2933

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	1m 34s
derive (1.71.0)	24s
build (1.71.0, wasm32-unknown-unknown)	1m 39s
derive (stable)	22s
build (stable, thumbv7em-none-eabi)	1m 27s
minimal-versions	0s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.71.0, sudo apt update && sudo apt install gcc-mult...	10m 53s
build (stable, wasm32-unknown-unknown)	1m 31s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	10m 52s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.71.0)	10m 45s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	9m 54s
	49m 21s

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