der: add `BmpString` #613

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 43s
build (1.65.0, wasm32-unknown-unknown)	1m 36s
build (stable, thumbv7em-none-eabi)	34s
build (stable, wasm32-unknown-unknown)	46s
test (1.65.0)	7m 11s
test (stable)	4m 35s
minimal-versions / minimal-versions	5m 50s
	22m 15s

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