der: conversions between `BitStringRef`/`OctetStringRef` and `[u8; N]` #2360

Job	Run time
build (1.81.0, wasm32-unknown-unknown)	54s
build (stable, wasm32-unknown-unknown)	51s
build (stable, thumbv7em-none-eabi)	55s
build (1.81.0, thumbv7em-none-eabi)	52s
minimal-versions	0s
fuzz	2m 15s
test (stable)	2m 27s
test (1.81.0)	2m 37s
	10m 51s

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