der

der: add `SetOfRef` struct #4482

Job	Run time
test-miri (x86_64-unknown-linux-gnu)	55s
build-no-std	2m 37s
test-cross (powerpc-unknown-linux-gnu)	2m 22s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1s
test (aarch64-apple-darwin, stable, macos-latest)	30s
test (x86_64-pc-windows-msvc, 1.85.0, windows-latest)	33s
minimal-versions / minimal-versions	3m 7s
test (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	1s
test (aarch64-unknown-linux-gnu, stable, ubuntu-24.04-arm)	40s
test-wasm	56s
test-derive (stable)	27s
test (x86_64-unknown-linux-gnu, stable)	30s
test (x86_64-pc-windows-msvc, stable, windows-latest)	32s
test-cross (armv7-unknown-linux-gnueabi)	2m 7s
test (aarch64-apple-darwin, 1.85.0, macos-latest)	12s
test (x86_64-unknown-linux-gnu, 1.85.0)	28s
test-miri (s390x-unknown-linux-gnu)	1m 12s
test-derive (1.85.0)	36s
test-careful	1m 26s
test-heapless	36s
test (aarch64-unknown-linux-gnu, 1.85.0, ubuntu-24.04-arm)	1s
	19m 49s

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