cms

der: add `SetOfRef` struct #3018

Job	Run time
build (stable, thumbv7em-none-eabi)	35s
build (1.85.0, wasm32-unknown-unknown)	32s
test (1.85.0)	1m 30s
test (stable)	1m 26s
build (1.85.0, thumbv7em-none-eabi)	29s
build (stable, wasm32-unknown-unknown)	26s
minimal-versions	0s
	4m 58s

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