der: add `UnusedBits` newtype for `BitString` (#1956) #3519

Job	Run time
build (stable, wasm32-unknown-unknown)	16s
minimal-versions / minimal-versions	34s
build (1.85.0, wasm32-unknown-unknown)	21s
build (stable, thumbv7em-none-eabi)	22s
test (stable)	36s
test (1.85.0)	50s
build (1.85.0, thumbv7em-none-eabi)	16s
	3m 15s

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