der: conversions between `BitStringRef`/`OctetStringRef` and `[u8; N]` #2362

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	51s
build (stable, wasm32-unknown-unknown)	55s
build (stable, thumbv7em-none-eabi)	53s
build (1.81.0, wasm32-unknown-unknown)	55s
test (1.81.0)	2m 30s
fuzz	2m 8s
test (stable)	2m 29s
minimal-versions	0s
	10m 41s

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